THE BARNET
BOOK OF

PHOTOGRAPHY.

THE BARNET
BOOK OF

PHOTOGRAPHY.

A COLLECTION OF PRACTICAL ARTICLES

BY

Capt. W. de W. ABNEY, C.B., F.R.S., Etc.
CHARLES H. BOTHAMLEY, F.C.S., F.I.C.
CHAPMAN JONES, F.C.S., F.I.C.
HAROLD BAKER
A. HORSLEY HINTON
JOHN H. AVERY
W. THOMAS
ANDREW PRINGLE
JOHN A. HODGES, F.R.P.S.
Rev. F. C. LAMBERT, M.A.
W. ETHELBERT HENRY, C.E.
JAMES PACKHAM, F.R.P.S.
THO'S. S. SKELTON

THIRD EDITION.

Published by
ELLIOTT & SON, BARNET, HERTS.
Percy Lund, Humphries & Co., Ltd.,
3, Amen Corner, London, E.C.
—
1898.

CONTENTS OF
THE BOOK.

PREFACE.

The purpose of this book is to place in the hands of every Photographer instructive articles on essential processes and manipulations, by eminent writers who have given such subjects their especial study, and who have borne in mind that whilst the experienced Amateur and the Professional may each find much to learn from a comparatively elementary description of methods and means, it is the Beginner who stands in greatest need of help.

In the mind of every photographer the name of Barnet is inseparable from a great Photographic Industry, and now it is intended that the name shall be associated with a good and useful book, which is called the Barnet Book of Photography, and it is left to the reader to say if the fulfilment of its purpose and the manner of its doing are such as to justify its existence.

To all who are interested in photography, who love it for itself and for its productions, and who desire to improve their own practice of its many processes and applications, this Book is respectfully dedicated.

Barnet, Herts. ELLIOTT & SON.
April, 1898.

COPYRIGHT.NEGATIVE BY W. L. F. WASTELL.
A Famous Pike Stream.
Contact Print on
Barnet Platino-Matt Bromide Paper.

Alpine Photography.

Writing in London on a day in winter with a murky sky and sloshy streets, the title of Alpine Photography is verily refreshing. It brings back days of sunlight and joyous experiment, and as we write the soul stirring scenery is before us called up by photographs taken under varying conditions of comfort and discomfort. That there is something different in Alpine photography to photography in our own country, we are bound to believe, since a special article is demanded for it.

The first question invariably asked is as to the nature of the outfit required. We should here like to divide our reply into two divisions. The one concerning the mountaineer, and the other the ordinary tourist. For the former we have no doubt in our minds that a hand camera to take ¼ plate or 5 × 4 pictures is the most convenient form of camera to take. It is not our business to advertise any person's wares and we shall content ourselves by saying that personally we prefer a camera which has separate slides and does not possess a magazine, more particularly when glass plates are to be used, though a form of Kodak is not to be despised. But perhaps we are prejudiced in favour of glass plates, for they are simple to manipulate and have no cockles nor other drawbacks which the careless photographer may have to encounter. Probably the most useful lens to employ is a doublet of which the focal length is about a quarter more than the width of the plate, since it includes a fair angle and the margins of the photographs are not likely to be markedly different in general density to the centre, as is the case when wide-angle lenses are employed. In England a lens which will cover with a large stop, say f/8, is a desideratum, but in the Alps it is very rarely that such a large ratio of aperture to focal length is required. As a rule for ordinary plates a lens has to be stopped down to f/16 to give a negative in say ¹/₅₀th of a second. Nevertheless where orthochromatic plates are to be employed it is very necessary to have a lens which will cover a plate satisfactorily with f/8 in order to use a colour screen for producing orthochromatic effects, since the loss of photographic light caused by the screen can only be compensated for by such an aperture even when the shutter is slowed down. The reader is therefore recommended on the whole to furnish himself with one of the modern lenses which work at f/8, though he must remember that the larger the aperture employed the more the margins and centre of the picture will suffer from unequal exposure. With some hand cameras there is a means of attachment to a stand, but a stand on a mountain is difficult to use and moreover has on more than one occasion been proved dangerous to carry. The mountaineer if he desires to give a time—and not an instantaneous—exposure on his excursion, would do well to have a small clip ready to attach to the head of his ice axe. The axe will form a sufficiently stable stand for the more prolonged, but still short, exposure that he may be required to give on some particular subjects such as a photograph at sunrise or near sunset.

Photographers in England are rarely afflicted with breathlessness through exertion, but it is different in mountaineering. A mountaineer may keep his wind, but it would be rare to find that his heart was beating equably after some spurt of exertion, such as rock climbing. It is often after some such exertion that he comes upon some view which he may wish to record on his photographic plate. The usual method of holding the hand camera would under such circumstances prove a failure so far as sharpness of image is concerned. Pressed against his "middle" or "upper" chest, the beatings of the heart will record themselves on the photograph. Under such circumstances resort must be had to some form of support on which to rest his camera. After many years' experience, the writer has come to the conclusion that there is no support superior to the ice axe. It is not necessary to cause it to stand upright in the ground, ice, or snow, though this should be done if possible. It will suffice to rest the point on the rock, and place the camera on the axe head, with the pick parallel to the body. We then have a firm support in one direction, and the hands, which are not affected by the automatic motion of the heart, can be trusted to keep it steady in the other direction. Photographs taken with a good lens, and with such a stand, will bear enlarging up to 22 inches, at least. It is because these photographs will bear enlarging that a small plate is recommended to the mountaineer. There is not a large proportion of Alpine views taken on the mountain side of which one would care to have anything but a memorandum, and it is such a size as that recommended which gives such a memento, and which, if desired, allows a more formidable size to be acquired at home, where we may suppose there are all the conveniences that a photographic laboratory affords. The writer has had experience on mountains with cameras varying from 12 × 10 to the ¼ plate size. When younger and more inclined to waste a few valuable minutes of daylight in putting up a camera stand, the 12 × 10 gave pictures which we often lamented having taken, whilst in his more mature years, a snap-shot has never been regretted. The cameras which require stands, require one porter at least to carry them, for although the late Mr. Donkin carried his own 7½ × 5 camera up the highest peaks, it is few men, who, even if they had the energy or the physique that he had, would imitate his example. A porter means an extra expense in fees, and an extra mouth to feed, and very likely entails slowness in a climb through having an additional man upon the rope. A quarter plate or a 5 × 4 camera the owner, however, can himself carry; but the best form of attaching it to his body has been a difficult task to evolve. Many and many different attachments have been tried. One thing is quite certain, and that is, the camera should be in a stout case, but it cannot be carried over the shoulders by a strap as we can do in comparatively level countries. Let anyone try to come down a rock with the camera slung over his shoulders, and he will soon find it dangling in front of his stomach, or swinging like a pendulum, and threatening to displace him from what at best may be a treacherous handhold. The method of attachment we adopt now, will be readily seen from the diagram.

The shoulder strap is utilized, but a ring is attached to the back of the case as shown, and a strap or piece of whipcord comes over the strap as shown. The two shoulders are in AA and the case is carried as a knapsack. The length of the cord or strap BB is so adjusted, as is also the length of the shoulder strap, that the camera lies against the small of the back, and that it will not swing away from the body. At one time the ring was placed in front of the case, but the result was merely to cause the top of the case to rest against the back. The plan shown above has answered under almost every variety of circumstances, and the weight is inconsiderable. (A friend has his camera attached to the bottom of a small "rücksac" and this answers, but as the writer does not carry his own provisions or change of garments he has not adopted this plan). A long day's march may be undertaken if this contrivance be employed, and the weight is scarcely felt.

For those who have not had extensive practice with hand cameras, a view finder is, if not a necessity, at all events, a great help. On the whole, perhaps the best form is that in which a miniature view falls on a ground glass. It must be recollected, however, that each view finder is adapted for some particular focal length of lens. The view in the finder and on the plate should be compared, and if the former is more extensive, the surplus ground glass should be covered up with a black mask.

If it be determined to take a camera with its stand, very few directions are required beyond those which apply to ordinary view work on the plains. It may perhaps be as well to mention that a camera stand placed on ice or snow, is not immovable until the iron shoes of the legs attain the temperature of the surrounding snow or ice. An exposure of a few seconds will often show an image which has moved on the plate.

The next point that we may call attention to is the plate to be employed. With a hand camera there is no absolute necessity to have the most rapid plate, as far as exposure is concerned, but in mountain work it must be recollected that there are very great contrasts to represent on the print. "The slower the plate the steeper the gradation" is almost axiomatic, and it must be recollected that only a certain amount of opacity will print if the deepest shadows only are to be kept of the greatest black obtainable in a print. It is evident that the greater the range of light and shade that is obtained of a printable density, the more true to nature the picture will be. For this reason a quick plate with a moderate gradation is to be preferred—as being most generally useful—but it should be a plate which is absolutely free from fog, and it should also be of as fine a grain as possible, the size of which has something to do with development. This is still more true when a camera stand and hand exposures are made. With a slow plate with feebler intensities of light, which must be the case when the lens is stopped down to admit of hand exposures, the gradation becomes more steep than if a fairly bright light be employed. A quick plate does not suffer in the same way, however small the stop may be. It has already been stated that isochromatic plates may be employed with a hand camera. For ice and snow views there is not much to commend their employment, unless to give a deeper shade to the sky and to the vast crevasses which so often form part of the foreground. The darker sky allows faint clouds to be visible in a print when they otherwise would be absent. Pictorially thus the isochromatic plate has something to recommend it. Celluloid films have often been substituted for plates by the writer, and excellent photographs have been obtained on them when they were fairly rapid. There is not much to be said in their favour as regards weight, for in most cameras the support for them weighs nearly as much as the glass plate. There is also a disadvantage in developing them, for they are not so easily manipulated as a rigid body. For convenience in travelling, however, they are to be highly commended. A gross of cut films do not weigh so much as a dozen plates and occupy much less space in the baggage. The question of the use of a Kodak camera with its roller slide, has not been brought forward, not because excellent results cannot be obtained with it, but simply because the writer prefers to use plates and films which can be got at any time for the purpose of development.

For travelling on the continent, and to one's mountain destination, experience has shown that a small hamper is the safest receptacle of all the necessary kit. A hamper which will contain two camera cases side by side is really sufficient; but it should be a little greater in depth. It may be thought that two cameras are to be taken, but such is not the intention. If a zinc trough be made of the size of one camera case it will contain all the developing apparatus necessary, the lantern, and the plates or films, and all the few etceteras which go to make one happy. (A screwdriver, a file, and some extra screws, and gummed paper and white blotting paper cut to the size of the plates should be enough for the etceteras). The hamper may be arranged so that the camera and view finder may be taken out without any derangement of the rest of the articles in it. The developing bottles and cups, with the dishes, may be similarly extracted. This prevents undue trouble in unpacking and packing. One grand thing to remember is, pack well but not distressingly tightly, in other words don't employ an expert packer if you wish for comfort. Have the hamper a size too large rather than a size too small. Also be it remembered that it is useless to stopper the bottles with all sorts of devices at home, and have to pack in an ordinary manner when once the contents of the hamper have been brought into use. Have your bottles covered with an indiarubber cap which can easily be removed and replaced; of course we are assuming that development is to take place during one's travels, and not to be left over for home. Personally we think that a speedy development after a view is taken will give the best picture. It may often happen that an undeveloped sensitive plate or film will suffer by its travels. There will or may be scratches and what not, which would be absent if the negative is finished at the time. The outfit for development which need only be taken is as follows: four developing dishes, bottles or cartridges of the dry developer, ammonia diluted to half its strength in a glass stoppered bottle (if in a wooden case, as for medicine bottles, it will be a further protection), a couple of tins of hyposulphite pounded up before the journey, carried in small tins (such tins as the half-plate platinum paper comes in are very convenient), two or three empty six ounce medicine bottles with good corks, a two or four ounce measure, a washing rack with a trough (there is a folding rack in the market which answers admirably; it has v shaped grooves which never damage the edges of the film, and one rack will take twenty-two glasses back to back). A zinc trough can be made to cover the plates with water when in the rack, a lantern (by preference a paper folding one), a dusting brush, a couple of dusters, and blotting paper cut into squares the size of the plates, with which to pack them—it is useful also to have spare pieces of blotting paper to place beneath the plates when drying, also a piece of mackintosh to place on the wash stand during developing operations—an empty pint wine bottle will be got at any hotel and in this the hyposulphite can be dissolved. The list looks formidable but the whole can be readily packed in the hamper of the size given. It will be seen that no intensifying solutions are enumerated amongst the requisites. A negative is better strengthened in the quiet of one's dark-room at home.

HOMEWARDS.
KARL GREGER.

Now we must give a hint or two as to the exposures required. We will suppose that on the plates to be used a satisfactory negative of an open English landscape, on a bright June day with fleecy clouds in the sky, can be secured with an aperture of f/11 in ¹/₂₅th of a second. If that be so, then on an equally fine day in July or August, at an altitude of about 6000 feet, the same kind of view should theoretically be secured in ¹/₅₀th second, and a stop of f/16—that is, the photographic light is about four times as strong. It must, however, be recollected that at this altitude, and particularly near mid-day, the shadows are not illuminated to the same degree from the sky. The darker blue sky shows that the light which at a low altitude goes to make a pale blue sky is to be found in the direct rays of the sun, and not scattered to give a luminous sky. As the shadows are principally illuminated by the light from the sky, it follows that the shadows will be darker at a high than at a low altitude, for this reason amongst others, the exposure should not be curtailed to the amount given above. If the aperture be reduced to f/16 it is probable that the exposure of ¹/₂₅th second will be not more than sufficient to give. For our own part we prefer to give longer and to expose well for the deep shadows, trusting to development to give us properly "gradated" pictures. As the sun goes down toward the horizon, the shadows get more illumined from local reflection, and it is scarcely necessary to alter the exposure until considerably nearer sunset than at home, when the exposure must be considerably prolonged. For views in which there is little but ice and snow, the exposure should be very much curtailed. There is so little contrast that if the exposure be at all prolonged the picture will be inevitably flat. The shadows are illumined by an immense quantity of light reflected from the white surface, and the difficulty is to get sufficient contrast. The writer well remembers one set of beautiful views, taken from the top of a mountain some 10,000 feet high, where the eye could see nothing but snow-fields and ice and swirling masses of clouds. The day was not bright, but to get a satisfactory picture a stop of f/32 was necessary with only an exposure of ¹/₇₀th of a second. Plates given an exposure of ¹/₂₅th second with a stop f/16 showed little besides a plain white mass. It would be difficult to give hints for every kind of view. The judgment of the operator must be brought into play and no actinometer will be of much use under the varied conditions which are the rule, not the exception.

Now as to development. The "one-solution" given by the metol and amidol cartridges are the most readily prepared, and in five times out of six will scarcely be bettered, but for the sixth time may fail, because of their "rigidity." For these exceptional negatives, solutions of an oxidizing agent such as pyrogallol, of a restrainer (bromide), and of an accelerator are to be recommended. For the latter, the carbonate (not the bicarbonate) of potash is much to be recommended, though some prefer ammonia. Two formulæ are given, either of which will be found extremely useful. When the exposure has been prolonged enough for details in deep shadows to be brought out, it will generally happen that over-exposure has been given to the high-lights, and it is to keep these in the printing state that care is required. In the old collodion dry plate days, it was very usual to bring out a complete phantom image of a subject before any density was given to it. When this was properly out, the intensifier of silver nitrate and pyrogallol was applied, and the picture gradually brought up to printing density. It was usually full of detail in the high-lights and shadows, all of which would be found in the finished print. Such is the same procedure which we recommend, strive to get out an image of feeble density but full of detail, and then give the density.

The plate should first of all be thoroughly soaked in a solution of the alkali which can be used, and then a few drops of the pyrogallol solution be dropped into the developing cup with an equal number of drops of the restrainer. The alkaline solution is then returned to the cup and again poured into the dish and over the plate. By degrees the required phantom image will make its appearance, and now bromide and pyrogallol are added until it is evidently complete. The plate is then washed in water, a final wash being given in a very weak solution of acetic acid or citric and water. After a final rinse with water the plate is treated with the pyrogallol solution and restrainer in the proportion recommended for the ordinary development of the plate, omitting the alkali. The density will begin to appear, and when it flags, a little alkali is added (a few drops at a time) to the solution. Keep the image fairly feeble at above half the proper printing density, and fix. The plate should then be kept for intensification, preferably by Mr. Chapman Jones's, when a mercury solution is applied, and then a ferrous oxalate to reduce the latter to the metallic state. It will be found if this procedure is adopted, that the negative is built up with a greater range of light gradation than by bringing it out by a one-solution method of development. If one wishes to exercise artistic treatment, then in the preliminary stage more importance can be given to any desired part by applying a camel's hair brush soaked in normal pyrogallol solution with its restrainer. The prominence thus gained will be kept in the subsequent operations. When applying the brush care must be taken that the image blends as it were with the rest of the picture. No abrupt increase of density must be permitted, as if it be, the result will be anything but satisfactory.

The following is an ammonia-pyro developer, with which the writer usually works.

(Of course, should the ammonia be taken half strength allowance must be made for the dilution.)

(When travelling it is very convenient to have the bromide weighed out into 20 grain packets.)

When the view has strong contrasts and the plate has been exposed for the shadows take of A 30 minims and 2 ounces of water and soak the plate in it as given above. Then add to the cup, of B 2 drams, of S 1 dram, and about quarter grain of P. Pour back the solution of ammonia from the dish, and then apply the mixture till all detail appears, and proceed as indicated above. A saturated solution of potassium carbonate may be substituted for the ammonia solution.

Before closing this chapter it may be of use to the reader to tabulate the number of thicknesses of atmospheres through which light has to travel at different altitudes of the sun at sea level.

If sunlight outside the atmosphere be represented by 1 and say ¹/₁₀th be cut off by 1 atmosphere, then after transmission through 2 atmospheres only .81 will reach the spectator, and if through 3 only .729. For any atmosphere the diminution will be ¹/₁₀th, that is, it will be .9^x where x is the number of atmospheres.

If we ascend the factor varies, there are less thicknesses of atmosphere to go through and we get the following table.

This table and the preceding one will enable a calculation to be made as to the exposure to be given. Thus at sea level with a photographic brightness of sun of 639,000 candles when nearly overhead, it will at 5° above the horizon only have a photographic brightness of about 1000. At about 9000 feet high the photographic brightness would when the sun is overhead be about 800,000 candles, and at 5° it would have a value of 350,000, showing the greater penetration through the thinner atmosphere.

W. de W. Abney, C.B., F.R.S., etc., etc.

Negative Making.

DEVELOPMENT, INTENSIFICATION,
REDUCING, Etc.

When a sensitive plate has been properly exposed under ordinary conditions, there is no visible change. The action of light produces what is known as a latent image or developable image, and in order to convert this into a visible image with sufficient opacity to be useful for printing purposes, it must be developed. In the operation of development, the plate is treated with some solution that will act on the exposed parts of the sensitive film and reduce the silver salts contained therein to metallic silver, in quantity proportional to the amount of light-action, whilst at the same time it produces no appreciable change in those parts of the film on which light has acted the least or not at all, and which correspond to the darkest shadows of the object that has been photographed. The solution used for this purpose is called the developer.

DEVELOPERS—GENERAL.

The substances that can be employed as photographic developers are now somewhat numerous, but the most useful for negative making are pyrogallic acid (also known as pyrogallol, or for brevity as pyro.), ortol, metol, and hydroquinone (also known as quinol). Ferrous oxalate is likewise used in special circumstances, but not for general work. An ordinary developer as mixed for use contains:—

1.—One of the above-mentioned substances (pyrogallic acid, ortol, metol, quinol) which is the actual developing constituent, and is known as the reducer, but requires the addition of the next constituent before it can work.

2.—An alkali, which may be sodium carbonate, potassium carbonate, caustic soda, caustic potash, or, if pyrogallic acid is used, ammonia. The alkali sets the reducer in action and is called the accelerator.

3.—A soluble bromide, which must be potassium bromide except when ammonia is used as the alkali, and then it may be ammonium bromide. The chief use of the bromide is to retard the action of the developer, and in particular to prevent its affecting those parts of the film that have not been acted on by light. For this reason the bromide is called the restrainer or, sometimes, the retarder.

4.—A sulphite, the function of which is to prevent the solution from becoming strongly discoloured and consequently staining the film. It also affects the colour of the reduced silver that forms the developed image, this colour being browner, and consequently of higher printing opacity, the lower the proportion of sulphite present. Sodium sulphite and potassium metabisulphite are the most commonly used.

The composition of a developer has to be so arranged that, whilst reasonably rapid in its action, it is not so rapid as to be beyond control, and does not produce "general fog" by acting on those parts of the film that have not been acted on by light.

AMONG THE ALPS.
CAPT. W. DE W. ABNEY, C.B., F.R.S. Etc.

DEVELOPMENT—GENERAL OPERATIONS AND PHENOMENA.

A developer is usually compounded immediately before use by mixing two or more solutions, and in order to ensure uniform action it is essential that the constituents should be thoroughly mixed before the liquid is applied to the plate. If the measuring or mixing vessel is large enough, this can be done by agitating the liquid; if not, the liquid may be poured once or twice from one vessel to another.

The quantity of developer necessary for a plate of a given size depends in some degree upon the size and character of the dish that is used, and is smallest when the bottom of the dish is quite flat and has no ridges or grooves. It is false economy to use too small a quantity, and it may be taken that for a quarter plate 1½ oz., for a half plate 2½ or 3 oz., and for a whole plate 4 oz. of developer should be used.

Ebonite, xylonite, or papier maché dishes are the best for all operations connected with negative making, since they are not so liable as porcelain or earthenware to break a plate if it is allowed to drop into them.

When applying the developer to the plate it is important to cover the whole surface of the plate rapidly and in such a manner as to avoid the formation of air bubbles, and the best way is to begin to pour on the developer at one corner of the developing dish and whilst pouring somewhat quickly move the vessel rapidly but steadily along the edge of the dish to the other corner. If there should be any froth or air bubbles on the surface of the developer, the last portions should not be poured out of the vessel into the dish, and then the risk of air bubbles forming on the surface of the plate will be lessened.

Sometimes after the developer has been poured on and the plate seems to be uniformly wetted, the liquid will recede from one corner or one edge of the plate and the part thus left uncovered will appear as a patch of lower opacity when the negative is finished. This happens either because the dish is not standing level on the table or because the bottom of the dish is not flat; sometimes it happens because too small a quantity of developer has been used.

After the plate has been covered by the developer the dish should be carefully rocked from time to time, and, for reasons that will be explained presently, the time required for the first appearance of the image and the manner in which the different parts of the image follow one another, should be carefully observed.

If the plate has been correctly exposed, the brightest parts of the image will appear (as black, of course,) in about a minute, more or less, according to the temperature, composition of the developer, and character of the plate, and the other parts will follow steadily in the order of their brightness, after which the image as a whole will continue to gain vigour or opacity up to a certain limit. The essential point is that the principal details in the deepest shadows of the subject shall appear and acquire a distinct printable opacity, before the highest lights become so opaque that the details in them are no longer distinguishable. Whether this condition is realisable or not depends very largely on the exposure that the plate has received.

If the image appears in considerably less than a minute and the different parts follow one another very quickly, the plate has been over-exposed, and the degree of over-exposure is indicated by the rapidity with which the image appears. In this connection it ought, however, to be stated that with metol and certain other developers, even when the plate has been correctly exposed, the different parts of the image appear almost simultaneously, though the first appearance may not begin until about a minute after the developer has been applied to the plate. It follows that with these developers it is difficult to recognise over-exposure, but it so happens that they are not suitable developers to use when there is any probability that the plates have been over-exposed. On the other hand, if the image is slow in appearing and the brightest parts of the subject are not followed in due course by the middle tones, the plate has been under-exposed, and there is considerable danger that the high-lights may become quite opaque before any details have appeared in the shadows, or even, in extreme cases, in the lower middle tones, that is to say, in those parts that are next in darkness to the shadows.

When it is desired, as it frequently is, to alter the composition of the developer during development, the substance or substances to be added should be put into the measuring or mixing glass, the developer poured out of the dish into the glass, and the well-mixed liquid poured over the plate as before. Any attempt to add substances to the developer whilst it is in contact with the plate will probably result in uneven action.

It should be borne in mind that temperature has an important influence on development, the time required for the first appearance of the image and for the completion of development being, as a rule, less the higher the temperature. Further, if the developing solutions are very cold, it is often almost impossible to obtain sufficient opacity.

Perhaps the most difficult thing in connection with development is to know when to stop the process, that is to say, when the image has acquired sufficient opacity, or "density," as it is often called. After all the required detail has become visible, the plate from time to time is lifted carefully out of the developer, allowed to drain for a moment or two, and then held between the developing lamp and the eye; the opacity of the image, especially in the highest lights and deepest shadows, being carefully scrutinised. The appearance of the image as seen when looking at the back of the plate, is also carefully observed.

For this purpose it is very much better that the light of the developing lamp should pass through transparent glass (ruby or deep orange) so that the flame itself is distinctly visible, instead of through ground glass or a coloured translucent fabric. Further, the flame of the lamp, whether gas or oil, should always be turned up to the same height, for it is clear that if the brightness of the flame used for making the examination is not fairly constant, all sorts of variable results will be obtained. For this reason it is much better to judge the opacity of negatives by artificial light than by daylight, the intensity of the latter being so variable. A paraffin lamp with a circular wick and a deep ruby chimney with a metal cap at the top, answers admirably.

No general rules can be laid down; the appearance of the properly developed image depends on the thickness of the film, the granularity of the silver salt, the presence or absence of silver iodide, and the composition of the emulsion used. Experience only is of value, and the best way to secure uniformly satisfactory results, is to keep as far as possible to one brand of plates. With some plates, for example, very little of the image should appear at the back of the plate, with others the greater part of the image must be distinctly visible there.

Sometimes, especially when using small sizes of plates, it is not easy to tell whether all the necessary detail in the shadows has been brought out, and this is an important matter, for if the small negatives are to be used for making enlarged negatives or prints, or lantern slides, there should be very little clear glass indeed even in the deepest shadows of the subject. As a rule it may be said that when every part of the image is at least gray the maximum possible amount of detail has been brought out. If the greyness begins to spread to the margins of the plate where it has been protected by the rebate of the dark slide, general fog is being produced, and, as a rule, little will be gained, but much may be lost, by continuing the development for any considerable time after this is observed. When development is completed the developer is poured off, the plate is well rinsed under the tap or in two or three changes of water, and is then ready for fixing.

DEVELOPMENT WITH PYRO-AMMONIA.

This method of development has the advantage that the constituents can be kept in concentrated solutions, considerable modifications in the composition of the developer can be made very readily and the negatives obtained are of excellent printing quality. On the other hand it cannot be satisfactorily employed with certain brands of rapid plates, because with them it has a tendency to produce general fog, and with some other plates, especially when they are old, it has a tendency to produce what is known as green fog.

Three solutions are prepared:—

[ [1] The metabisulphite is dissolved in about 8 oz. (80 parts) of water with the aid of heat, and the pyrogallic acid is then added. When the liquid has cooled it is made up to 10 oz. (100 parts) by addition of water, the whole being well mixed by shaking.

For each ounce of developer, take 20 minims of reducer, 20 minims of restrainer and 40 minims of accelerator, and make up to 1 oz. with water. With some plates 60 minims of accelerator and 30 minims of restrainer may be used, but any greater proportion of accelerator has considerable tendency to produce general fog. On the other hand the proportion of restrainer can often be increased with advantage since, unless the amount added is very large, its chief effect is to prevent general fog; 30 minims of restrainer to 40 minims of accelerator, or 40 minims of accelerator to 60 minims of restrainer are proportions that can be recommended. Too low a proportion of bromide should be carefully avoided.

It is very important to ascertain, by careful trial with each brand of plates that is to be used, what is the maximum proportion of ammonia that can safely be added, and what proportion of bromide to ammonia is necessary in order to prevent general fog. As a rule, the more rapid the plates the smaller is the quantity of ammonia that can be used with safety.

By far the best plan is to keep development well under control by adding only part of the accelerator at the beginning of development and adding the rest as circumstances require.

For each ounce of developer take 20 minims of pyro solution and make up to the required bulk with water. In another measure mix for each ounce of developer 40 minims of bromide solution and 60 minims of ammonia solution, and regard this as the maximum quantity that can be added with that bulk of developer. Now to the diluted pyro solution add about a quarter or one-third of the ammonia and bromide solution, pour this mixture on the plate and observe what happens.

If the mode of appearance of the image indicates that the plate has been correctly exposed, about half the remaining ammonia and bromide mixture may be added to the developer at once, and the action allowed to continue, with occasional rocking of the dish. If development proceeds satisfactorily and, in particular, if the chief details in the shadows begin to appear before the highest lights have become too opaque, it is not necessary nor advisable to add the last portion of the ammonia and bromide mixture, since the tendency to general fog and green fog is reduced when the proportion of ammonia is kept as low as possible. On the other hand, if the development flags and the appearance of shadow detail is a little tardy, the rest of the ammonia and bromide mixture must be added.

If the plate seems to be over-exposed, no more of the ammonia and bromide mixture should be added for some time, until it is seen whether the quantity already in the developer will suffice to complete development. If it seems that the over-exposure has been considerable, a further quantity of pyro solution (10 to 20 minims per oz.) and also of bromide solution (10, 20, or 30 minims per oz.) may be added with advantage. Development is then allowed to continue and the negative is examined from time to time; if it is seen that the opacity does not increase, or if sufficient detail in the deep shadows does not appear, further small quantities of the ammonia and bromide mixture may be added cautiously until the required result is obtained, waiting a little while to see the result of each small addition before adding more.

Teasels
By Carine Cadby.

When the plate behaves as if under-exposed, dilute the developer at once with half the quantity or an equal quantity of water, according to the degree of under-exposure indicated, and add the whole of the ammonia and bromide mixture. These modifications should check the rate at which the high-lights of the subject gain opacity, whilst accelerating the appearance of the middle tones and shadows. Should this effect not be produced, further quantities of ammonia and bromide mixture may be added or, in extreme cases, ammonia alone, and the developer may be still more diluted with water.

If any considerable parts of the image still show no detail, local development with a brush may be tried as a last resource. A soft camel's hair brush, preferably mounted in quill, is used. Some of the ammonia and bromide mixture is placed in a vessel and diluted with two or three times its volume of water. One corner or edge of the plate is raised so that the part to be treated is lifted out of the developer, the diluted ammonia and bromide mixture is applied rapidly with the brush, and the plate is allowed to drop gently back into the developer. The treatment may be repeated if necessary.

Should all these devices fail, the plate is hopelessly under-exposed.

Sometimes, when working with a diluted developer as just described, it happens that although all the necessary detail has been brought out, the image gains in opacity very slowly. Provided that all the required detail is visible, small quantities of pyro solution may be added in order to gain opacity more quickly.

PYRO-SODA DEVELOPMENT.

When sodium carbonate is used as the alkali in place of ammonia the developer acts somewhat more slowly and is less liable to produce fog, especially with very rapid plates, and there is very little tendency to produce green fog. On the other hand, variations are not so easily made in the composition of the developer. Some people find the absence of the smell of ammonia a decided advantage.

[ [2] No more of the dilute pyro solution should be made up than is likely to be used during the same day, but it will keep well enough for a day or two.

[ [3] The sodium sulphite and carbonate are dissolved, with the aid of heat, in about 8 oz. (80 parts) of water, the bromide added, and the liquid when cold made up to 10 oz. by adding water.

For use mix equal parts of dilute pyro solution and soda solution and pour over the plate.

If the exposure has been correct the image will begin to appear in about a minute, and development is then allowed to go on with occasional rocking of the dish, until the negative is sufficiently opaque.

If the plate behaves as if it were under-exposed, at once dilute the developer with an equal bulk of water and pour it back over the plate. If the high-lights continue to increase in opacity, but the rest of the image does not appear, add some more of the soda solution with or without some more water. Should parts of the plate still remain blank, apply some of the soda solution to them with the aid of a brush as described under pyro-ammonia (page 32).

If the rapid appearance of the image indicates that the plate is over-exposed, at once pour off the developer into a measure or mixing glass and rinse the plate well with water. Add to the developer a small quantity of potassium bromide solution (1 in 10 of water) which should be kept at hand for this purpose. A small quantity of pyro stock solution may also be added. The developer is then poured over the plate again. When the over-exposure seems to have been considerable, the amount of potassium bromide added may amount to 4 grains (or 40 minims of the 1 in 10 solution) per ounce of the developer, but this proportion should not be exceeded; even small quantities of bromide in the pyro-soda developer have a marked influence in retarding development.

When there is reason to suspect over-exposure, not more than half the soda solution should be added at the beginning of development, and the rest may be added or not, as the case may require.

[ [4]See foot-note to [page 30].

Mix equal parts of ortol solution and soda solution.

This developer behaves in much the same way as pyro-soda and gives very similar results. It has the advantage, however, that it does not stain the fingers, and has practically no tendency to produce either fog or stain on the plates. Moreover the same quantity of solution can be used for several plates; when the action becomes perceptibly slower or weaker, part of the old solution is poured away and an equal quantity of freshly mixed ortol and soda solutions is added.

The chief differences to be observed are (1) that the different parts of the image follow one another more rapidly than with pyro-soda, even though the plate may have been correctly exposed, and (2) the colour of the reduced silver is somewhat bluer than with pyro-soda, and therefore in order to obtain the same degree of printing opacity, as distinct from visual opacity, development must be carried a little further.

Apart from these differences, what has been said of pyro-soda holds good for ortol soda and need not be repeated.

Mix two parts of hydroquinone solution with one part of alkali solution and one part of water, or, if a more energetic developer is wanted, mix equal volumes of the hydroquinone and alkali solutions. Hydroquinone is not an advantageous developer for general purposes, but it is useful when negatives are required showing strong contrast between the highest lights and the deepest shadows, and especially when it is important that there should be no deposit at all in the deepest shadows. This is the case, for example, when copying line engravings, pen and ink drawings and similar subjects.

DEVELOPMENT WITH FERROUS OXALATE.

This method of development also is not well adapted for general work, but it is invaluable for certain purposes. The reduced silver has a pure grey-black colour and there is exceedingly little tendency to produce fog of any kind. On the other hand, the developer admits of little modification in its composition and therefore the exposure must be fairly correct. It is also important to avoid contamination with even minute quantities of hypo, since this substance very readily causes stains.

About three-quarters of the total quantity of water is mixed with a small quantity (not more than 50 minims per 10 ozs., or one part per 100) of sulphuric acid, and the ferrous sulphate (proto-sulphate of iron) which must be in clear pale green crystals without any yellowish incrustation, is dissolved in it with the aid of a gentle heat. After the solution has cooled, it is made up to the specified volume with water. This solution alters when exposed to air, and should, therefore, be kept in small (2 oz.) bottles, filled up to the neck and tightly corked.

For use take four parts of oxalate solution and one part of ferrous sulphate solution, pouring the latter into the former and not vice versa. In order to obtain slower action with a rather softer image and a slightly browner deposit, the developer may be diluted with an equal volume of water. Slower action, with slightly increased printing contrasts, and clearer shadows, results from an increase in the proportion of bromide.

FIXING.

After development is finished, the dark-coloured reduced silver that forms the image remains mixed with a considerable quantity of semi-opaque, yellowish unaltered silver bromide, which would not only interfere with the printing, but would also gradually darken when exposed to light. The negative must therefore be "fixed" by dissolving out the unaltered silver bromide, and this is accomplished by immersing the plate in a fairly strong solution of sodium thiosulphate (formerly called sodium hyposulphite) commonly known as "hypo." The usual strength of the fixing is as follows:

A solution of double this strength is, however, not unfrequently used, and acts more rapidly, especially in cold weather.

The developed plate, after being well rinsed with water, is placed in the fixing bath and allowed to remain in it with frequent rocking until the silver bromide has all been dissolved out of the film. This is ascertained by lifting the plate out of the dish and looking at the back by reflected light, the plate being held in front of something dark. It is not difficult to see whether the silver bromide has all disappeared or not, but in order to ensure complete fixing the plate must not be taken out of the bath as soon as this has happened, but should be left in for a few minutes longer, the dish being rocked so that the dissolved silver salt may diffuse out of the film into the fixing bath.

When removed from the fixing bath the plate should be allowed to drain into the bath for a few moments and should then be washed for five or ten minutes in running water under the tap. It is best to put the plate in a dish standing on the sink and have a piece of flexible indiarubber tubing reaching from the tap to within a couple of inches or so of the top of the dish, so that the water may not splash too much. After washing in this way, the plate is placed in a grooved zinc rack, which is immersed in a tank (preferably of zinc), containing sufficient water to completely cover the plates, and here it remains until the whole batch of plates in hand has been developed and they can all receive their final washing together. The plates stand upright in the rack, and the entrance and exit of the water must be so arranged that the water enters at the bottom and overflows at the top, or, what is perhaps better, enters at the top and is drawn off from the bottom, the waste pipe opening at the bottom of the tank and being bent and carried upwards until its mouth is at the level at which the water is to stand in the tank.

When running water is not available the plates may be washed in dishes. After being well rinsed to remove the adhering hypo solution, the plate is covered with water (about 3½ oz. for a half plate or 5 oz. for a whole plate) and allowed to remain with frequent rocking for five or six minutes. The water is then well drained off, a second quantity added and allowed to remain for the same time as before, with frequent rocking, when it is poured off in its turn. Treatment in this way with six successive quantities of water will remove all the hypo, provided that the film has not been treated with alum.

Another plan, rather less troublesome, but also less expeditious, is to place the rack containing the plates in a tank not much more than big enough to hold it, taking care that there is not less than two inches between the lower edges of the plates and the bottom of the tank. After standing for some time the rack and the plates are slowly and carefully lifted out and allowed to drain, the tank emptied and filled with fresh water, and the rack and plates then replaced. Eight or ten successive quantities of water applied in this way should remove all the hypo, but if there is any doubt on this point the plates, after they are supposed to be washed and have been removed from the tank, should be allowed to drain into a measuring glass or into a dish, the contents of which are afterwards transferred to a measuring glass and mixed with a small quantity of a solution of silver nitrate. If the plates are really completely washed nothing will happen, or at most a white precipitate will be produced which will remain white if not exposed to daylight. If, on the other hand, the plates still retain hypo, the silver nitrate will produce a precipitate which will gradually become orange and eventually dark brown. Should this happen, the washing must be continued.

Drying.—If the negatives are allowed to dry in the rack in which they were washed, the process is slow, and sometimes if the washing has not been complete, the middle portions of the negatives, which dry last, are less opaque than the rest. An excellent method of drying negatives rapidly and in such a way that no dust can fall on the film, is to drive nails (preferably of copper) into a wall or a board fixed against the wall, at distances apart depending on the size of the plates. Each plate then rests, with the film downwards, between a pair of nails, the lower corner of the plate resting against the wall, as shown on previous page.

WINTER TIME ON THE ALPS.
CAPT. W. DE W. ABNEY, C.B., F.R.S. Etc.

Alum Bath.—It is frequently recommended that all plates should be immersed in a strong solution of alum, for the purpose of preventing "frilling" by hardening the film. Its use is, however, attended with the great disadvantage that liquids diffuse into and out of a film so treated with much greater difficulty than in the case of an ordinary film, and consequently if the film is alumed between development and fixing, the fixing is not only much slower, but the washing after fixing requires a very much longer time. If, therefore, the alum bath is used at all, it should not be applied until after the film has been well washed after fixing. The following solution may be used:—

If a strong solution of alum is applied to the plate for a long time, the film may become so thoroughly hardened that it partially loses its adhesiveness, and there is a possibility that it will begin to peel from the glass after the negatives have been stored for some time in a dry place. The solution given above is quite strong enough and plates need not be immersed in it for more than five minutes, after which they must, of course, be again well washed. The hardening of the film, if not carried too far, no doubt makes it less liable to be injured by abrasion and the like.

Frilling.—It sometimes happens that during the various operations of development, fixing and washing, the film begins to leave the plate and rise in puckers along the edges. This is known as "frilling," and in bad cases it may spread until a large part of the film has detached itself from the glass. It is due to excessive or irregular absorption of water by the gelatine, and at one time was commonly met with, but it rarely occurs with the dry plates of the present day. It is most likely to arise if there is any considerable difference of temperature between the various liquids and the wash-water, or during very hot weather when all the liquids are much warmer than usual.

When frilling does occur, the plate must be treated carefully, so as to avoid tearing the film, but unless it is very bad and shows a tendency to spread, all the operations, including washing after fixing, should be completed before any special measures are taken to remedy the defect. On the other hand, if the frilling spreads rapidly, the plate should be carefully rinsed two or three times with water and placed for five minutes in the alum bath, with occasional gentle rocking, after which it is again well washed to remove the alum, and the various operations are completed. There is one exception to the procedure just indicated; if the frilling becomes bad while the plate is being fixed or during washing after fixing, the alum must not be applied until the fixing and the washing after fixing are completed. If something must be done in these circumstances, the plate, after draining, but without any previous washing, may be placed for about ten minutes in a saturated solution of common salt. It can afterwards be put back into the fixing bath, also without any intermediate washing, and the remainder of the process carried through.

Although the methods just described will check the frilling, they will not remove its effects. For this purpose the plate after its final washing is allowed to drain thoroughly and is then immersed in methylated alcohol, preferably of the old kind, though the new kind can be made to do. The alcohol abstracts water from the film, which consequently shrinks to its original size and can be pressed back with the fingers into its proper position on the plate. Should the film be opalescent it should be removed from the first quantity of alcohol and placed in a second quantity, after which it should be set up to dry. The plates should not remain too long in the alcohol or the gelatine will contract too much.

DEFECTS IN NEGATIVES.

A perfect negative presupposes a perfect plate, correct exposure, and correct development stopped at exactly the right time. It is almost unnecessary to say that all these conditions are rarely satisfied, and consequently most negatives fall more or less short of perfection. The defects may be broadly grouped under two heads, namely, those due to imperfections existing in the film before exposure, and those due to defects or errors in the way in which the plate has been treated. It will be more convenient to deal with the latter, and larger, group first, but there is really no hard and fast division between them.

The Negative is Thin, or in other words, whilst showing good gradation, and sufficient relative contrast between the different parts, is as a whole lacking in opacity or printing strength, and gives prints that are deficient in vigour and contrasts. The plate has been removed from the developer too soon, and the remedy is to intensify the image (see [p. 51]). Sometimes the want of opacity is due to the fact that the developer was too cold.

The Negative is too Dense or Opaque and consequently although showing good contrasts and gradations, takes a long time to print, especially on dull days. The developer has been too energetic, or development has been continued too long; the remedy is to reduce the image (see [p. 50]).

The Image is "Flat," or shows comparatively little contrast between the highest lights and the deepest shadows. This may, of course, be due to the absence of contrasts in the subject photographed; it is commonly due to over-exposure; it may be caused by using a developer containing too little reducer, or restrainer, or both, and too much alkali; sometimes it arises from a defect in the quality of the emulsion, or from the fact that the plate has been coated with an abnormally thin film of emulsion.

The Image is "Hard," or shows excessive contrasts between lights and shadows, and is defective in the range of its half-tones. This is probably due to under-exposure, but may have been aggravated by the use of a developer containing too much bromide or too little alkali. Local reduction (see [p. 50]) may partially remedy the defect.

Fog.—A more or less marked grey deposit of reduced silver extends over the whole surface of the image. It may be due to over-exposure, in which case the edges of the plate that have been protected by the rebate of the dark slide usually remain clear. It may also be caused by using a developer containing too much alkali, or too little restrainer, or both, or by the plate having been exposed to actinic light outside the camera, including the light from the dark-room lamp if the glass or coloured fabric used as the screening material is not efficient. In any of these cases the defect would be observable up to the extreme edges of the film.

The character of the dark-room light should be tested by exposing one half of a plate to it at a distance of say nine or twelve inches for five or ten minutes, the other half of the plate being protected by some opaque substance. The best plan is to put the plate into a dark slide and draw out the shutter half-way. After exposure the plate is treated with a developer in the usual manner, and it can then be seen whether or no the light has exerted any action on the plate.

Slight general fog may as a rule be neglected, but if the amount of fog is at all considerable the plate should be treated with a reducer, and afterwards the image can, if necessary, be intensified.

Green Fog.—The surface of the film shows a peculiar brilliant green or yellowish-green lustrous appearance, generally in patches, when examined by reflected light, but is more or less distinctly pink when the plate is looked through. This effect is rarely observed except when pyro-ammonia has been used as the developer, and it most frequently occurs with old plates, especially if development has been long continued or has been forced by the addition of comparatively large quantities of ammonia.

If the green fog is only slight it does not affect the prints made from the negative, but in bad cases the prints have a patchy appearance and are less deeply printed at those points where the green fog is worst. Two methods are available for the removal of green fog.

In one of these the plate, after being fixed and washed, is placed in a hypo solution of half the strength of the ordinary fixing bath, and to this hypo solution is added a very small quantity of a solution of potassium ferricyanide, and the mixture is allowed to act on the plate for some time, the dish being rocked occasionally. The green fog will gradually disappear and some more of the ferricyanide may be added, if necessary, to secure this end, but it is important to keep the proportion of ferricyanide as low as possible, otherwise the image itself will be reduced. For this reason, if it is seen or suspected that the green fog is likely to be bad, development should be carried a little farther than usual in order to allow for the slight reduction that accompanies the removal of the green fog.

The other plan is to immerse the plate in a dilute solution of ferric chloride (perchloride of iron) until the green fog has been completely bleached, then wash, first in a dilute solution of oxalic acid and afterwards in water, and finally treat with a developer, preferably ferrous oxalate. The green fog is converted into a very fine grey deposit which is almost invisible and has no appreciable effect on the printing qualities of the negative.

Black Spots may be due to particles of dirt that have been allowed to lodge on the film during one or other of the operations, or during drying. They may also be due to particles in the emulsion, and in the latter case are generally round and sharply defined.

Black Marks of the nature of irregular streaks, looking, so to speak, like black scratches, are generally due to mechanical abrasion of the film. Pressure produces a developable image similar to that produced by the action of light.

Black Bands, indistinct or nebulous at the edges, are sometimes caused during the coating of the plate with the emulsion, in which case they, as a rule, extend all the way along or across the plate. More commonly they are due to defects in the hinges of the dark slides, which may produce the bands either by allowing light to pass through, or by giving off exhalations that affect the plates if they are allowed to remain in the dark slide for a long time. If the bands are due to the hinges, they will, of course, correspond with them in position, and if the hinge is double, in the distance between them.

Transparent Bands, or bands showing less opacity than the rest of the image, are sometimes caused by exhalations from the material forming the hinges of the dark slides.

Transparent Spots if small ("pinholes"), are generally due to the presence of particles of dust on the surface of the plate when it was exposed. Prevention lies, of course, in carefully dusting the plate and the dark slide with a soft, clean, dry camel's hair brush, before putting the former into the latter. If the spots are larger and circular, they are due either to the formation of air bubbles on the surface of the plate during development, or to the presence in the film of insensitive particles.

Uneven Opacity or Density, varying gradually from one end or side of the plate to the opposite end or side, is due to uneven coating of the plate. If there is a distinctly defined patch, less opaque than the rest, the plate was not properly covered by the developing solution.

Stains.—A uniform stain, of a yellowish or brown colour, is produced when the pyro developer contains too small a proportion of sulphite or is allowed to act for a very long time. Such a stain is rarely observed with the other developers mentioned above. The pyro stain can be more or less completely removed by immersing the plate for some time, with repeated rocking, in the alum solution given above, 1 drachm of sulphuric acid being added to every 10 ounces. The plate must afterwards be well washed in soft water. Similar stains in patches may be caused by using dirty dishes or a developer that has become turbid by being frequently used.

Deep Yellow-Orange or Brown Stains, appearing gradually in patches or all over the negative, some time after it has been fixed, and washed, and dried, are due either to imperfect fixing or to incomplete washing after fixing. There is no practicable remedy.

Dock
By Carine Cadby.

Halation.—When the subject photographed includes some part much more brightly lighted than the rest, such as a window in an interior subject, the details of the bright part are not only lost, but the image of it seems to spread in all directions, obliterating the details of the surrounding portions. The effect is especially noticeable when the subject includes dark parts which necessitate a somewhat long exposure. A window at the end of a long dimly lighted interior, or dark trees against a bright sky are cases in point. The effect is really due to the fact that the sensitive film is not perfectly opaque, and some of the incident light passes through the film and is reflected from the back surface of the glass on to the under side of the film, producing a blurred image superposed, as it were, on the normal image formed at the surface of the film by the action of the direct light. The effect is known as "halation." It is prevented by having a perfectly opaque film, which is a condition difficult to realize in practice, and which, moreover, introduces certain other disadvantages. It is also prevented by coating the back of the plate with some substance that will absorb the rays that have passed through the film, and so prevent their being reflected back against the under side of the film. The substance used must either be opaque or must have a deep orange, brown, or red colour, and it must have the same refractive index as the glass, otherwise the reflection will not be prevented. For practical convenience it must also be easily applied and easily removed. Many substances have been recommended but nothing is so good as caramel, prepared by the action of heat on sugar. In order to get the mixture to dry completely after it has been applied, a somewhat troublesome process of purification is necessary, but caramel specially prepared for the purpose can now be obtained from dealers in photographic materials. The caramel (which is a solid substance) is dissolved in just enough water to make a thick syrup, which is carefully applied to the back of the plates in a thin layer by means of a flat brush.

If the caramel does not dry properly the solution may be thoroughly mixed with about one quarter (or more) of its weight of very finely powdered burnt sienna or burnt umber, "ground in water."

After being coated, the plates require some time to dry, and must, of course, be carefully protected from light. If the dark-room is thoroughly dark, the plates may be put up to dry in the same manner as negatives (see [page 40]), but if the dark-room is not suitable, some sort of drying box must be used.

After exposure and before development the backing is removed with a damp sponge; if caramel only is used in a form completely soluble in water, it need not be removed unless a developer is being used that is to be applied to several plates in succession.

REDUCTION.

When a negative is too opaque or dense it must be reduced by dissolving away part of the silver that forms the image. The same process is also applied for the removal of general fog, sometimes with a view to subsequent intensification.

The simplest solution to use for this purpose is known as the Howard Farmer reducer and is a solution of hypo mixed with a small quantity of potassium ferricyanide (red prussiate of potash).

The negative which, if it has been previously dried, must be soaked in water for some time until it is thoroughly and uniformly wetted, is placed in some fresh hypo solution (the ordinary fixing-bath solution diluted with an equal volume of water) to which a small quantity of the ferricyanide solution has been added, and the dish is rocked repeatedly to ensure uniform action. The rapidity of the reducing action depends on the proportion of ferricyanide solution added, and it is very important not to add too much, otherwise the process gets out of control and reduction goes too far. The image should be carefully watched and the plate removed from the solution and rapidly washed before the apparent reduction is quite as great as it is intended to be. It is much better to stop too soon than too late, because if it is found that a little further reduction is necessary, the plate can be again immersed in the hypo and ferricyanide.

The ferricyanide reducer can be applied locally for reducing high-lights, halated windows, etc., and this is often very valuable, especially in the case of under-exposed negatives. A small quantity of hypo and ferricyanide solution is mixed in a measuring glass or some other suitable vessel. The plate is immersed in plain hypo solution in a white dish for a short time and is then raised by one corner or one edge until the part to be reduced is above the solution. The mixture of hypo and ferricyanide is carefully applied with a camel's hair brush to the parts that are too opaque, and after a few moments the plate is allowed to slip back into the hypo solution and the dish is rocked. If the reduction is not sufficient, the same proceeding is gone through as often as necessary. The reducer should not be allowed to act too long before putting the plate back into the hypo, otherwise the reduction may spread further than is desired. Further, the reducer must not be too strong (i.e., contain too much ferricyanide), otherwise it will produce brownish stains and the action may be too energetic.

The other reducer is known as Belitzski's reducer, and is made up as follows:—

[5] The formula in "parts" does not strictly correspond with that in ounces, but the difference is immaterial.

The constituents must be dissolved in water in the order given. The solution can be used at once and it keeps fairly well if protected from light, in well corked bottles filled up to the neck.

INTENSIFICATION.

Intensification is a process in which the opacity of the image is increased by adding some fresh matter, metallic or otherwise, to the reduced silver that constitutes the developed image.

The usual plan is to bleach the image by means of a solution of mercuric chloride (mercury perchloride or corrosive sublimate), which converts the dark-coloured silver into a white mixture of silver chloride and mercurous chloride, and this is subsequently treated with some re-agent which will reconvert the image into a dark product of greater opacity than the original.

It is absolutely essential to successful intensification that the negative be completely fixed and completely washed after fixing, for any trace of hypo left in the film will give rise to brown stains. It is also important, in order to prevent stains of another sort and to secure uniform action, that the mercuric chloride solution be mixed with a small quantity of hydrochloric acid. Too much acid will cause frilling. If the negative has been dried it must be immersed in water for, as a rule, not less than half-an-hour, in order that it may be thoroughly and uniformly wetted.

When uniform intensification is required the negative is allowed to remain in this solution until it is completely bleached. If, however, it is desired to intensify the shadows more than the high-lights, the plate should be removed from the solution as soon as the shadows have bleached, and should be rapidly washed in order to stop the action on the more opaque parts of the image.

In either case the negative must be thoroughly washed after bleaching, and the water used must be soft water. Hard water tends to produce a precipitate of the mercury salt in the film, which may subsequently lead to stain or fog.

Perhaps the best plan of all, when constant results are desired, is to treat the bleached negative with the ferrous oxalate developer, which will gradually convert the white image into a black one, after which the plate is thoroughly washed and dried. It is recommended that the first water used for washing should be slightly acidified with oxalic acid.

Instead of using ferrous oxalate the bleached plate may be treated with a weak solution of ortol or metol to which some sodium carbonate (soda crystals) solution has been added, but no sulphite. After the image has blackened completely the plate is washed.

With any of these methods if the first intensification is not sufficient, the plate may be again bleached with the mercury solution and the process repeated.

An old method, frequently used, is to treat the bleached plate with dilute ammonia, which converts the white image into a dark brown one of very considerable printing opacity. The results are often very good, but are somewhat uncertain, since the precise effect obtained depends on the strength of the ammonia solution and the time during which it is allowed to act. With somewhat strong ammonia, allowed to act for a fairly long time, part of the intensification first produced is removed. This affects the shadows more strongly than the lights and the result is to increase the contrast of the negative, which is very useful for certain purposes.

The negatives intensified with mercury solution followed by ammonia are more liable to spontaneous change and deterioration than those intensified with mercury solution followed by one of the developers. The latter, in fact, if properly washed, may safely be regarded as permanent.

Uranium Intensifier.—A very considerable degree of intensification can be obtained by the use of the uranium intensifier, which is very different in its mode of action, and is a little uncertain in its results. A solution containing potassium ferricyanide and a uranium salt, generally the nitrate, is applied to the negative, and a deposit of a deep orange-red colour is formed upon the silver image and very greatly increases its printing opacity. The great difficulty is to prevent this deposit forming on the whole of the film, and it is absolutely necessary that every trace of hypo should be washed out of the film. The addition of acetic acid to the solution not only promotes uniformity of action, but also helps to keep the shadows of the image clear.

The negative is placed in this solution and allowed to remain with occasional rocking until the degree of intensification is sufficient, which can only be learnt by experience. If it is seen that the deposit is beginning to form on the clear parts of the negative, the plate should be at once removed. After intensification the plates are well washed. If the water is "hard" the intensification will be slightly reduced during washing, and this is often useful in removing a slight stain over the whole of the plate. Treatment with water containing a small quantity of ammonia or sodium carbonate removes the whole of the deposit, but leaves the original image slightly reduced and also partially altered in composition.

VARNISHING.

A negative after been thoroughly dried may be used for printing without any further treatment, especially if only a few prints are required and the ordinary ready sensitized papers or emulsion papers are used. It is, however, better to protect the negative from mechanical as well as chemical injury by means of a film of hard varnish or collodion.

Many excellent negative varnishes can now be purchased, and the general mode of application is the same. The negative must be thoroughly dry, and in order to secure this and to make the varnish flow more easily, the negative is very carefully heated in front of a fire or over a small stove until it is just warm, but not hot. The negative is best supported by means of a pneumatic holder held in the left hand, and a fairly large pool of varnish (the exact amount can only be learnt by experience) is poured on the plate somewhat towards the right-hand top corner, and by carefully tilting the plate it is made to run first to the nearest corner, then along the edge to the further left-hand corner down to the nearer left-hand corner, and back to the right-hand bottom corner, from which it is poured into a bottle. The plate is gently rocked whilst it drains into the bottle, and as soon as the varnish ceases to drop the plate is again carefully warmed until the back of it is just too hot for the back of the hand to bear, after which it is placed in a rack to cool.

It is necessary that the varnish should be quite clear and free from any solid particles, and if necessary it must be filtered through a plug of cotton wool moistened with alcohol and placed in the apex of a glass funnel which is resting in the neck of a clean and dry bottle. Since dust may fall into the varnish whilst it is on the negative, it is the best plan to pour the excess of varnish off the negative into a second bottle instead of back into the first, out of which it was poured. To put it in another way, one bottle should be kept for the clear varnish, and a second bottle for the varnish poured off the plate. When the second bottle is full, its contents are filtered into the first bottle for use again.

Instead of varnish, a film of collodion, toughened by the addition of a few drops of castor oil, and known as "leather" collodion, may be used. The collodion is applied to the plate in the same way as varnish except that the plate is not warmed.

C. H. Bothamley.

Lenses.

Photographs of flat objects such as leaves, lace, drawings, etc., can be made by simply putting the object on the sensitive surface and exposing the arrangement to light. But this method will not serve if the photograph is wanted of any other size than the original, nor with solid objects of any size, except perhaps in the production of full-size profiles of faces. It is therefore quite the exception in photography to "print" directly from the object itself, and the only alternative is to produce an image on the sensitive surface.

All illuminated objects reflect light and so become for practical purposes sources of light, just as the moon shines, as we say, although it only shines because it is shone upon by the sun. The simplest source of light to consider is a point of light, and if we can get a dot of light on a white surface from a point of light we have at once an image of that point of light. The smaller the dot the sharper or more perfect is the image, the larger the dot the more diffused or fuzzy is the image. It is impossible by any known means to get the dot so small that it is an actual point, that would be absolute perfection, and on the other hand there is no size of the dot at which it can be definitely said that it ceases to be an image. Every point of an illuminated object is a point of light, and fine definition consists in keeping these points separate in the image. So far as the dots overlap they are confused. Confusion, or diffusion, or fuzziness is sometimes desirable, as for example in a portrait, which may be excellent although it is impossible to distinguish in the picture the individual hairs on the person's head.

Fig. 1.

The simplest means for getting an image is a small hole in an opaque screen. In fig. 1, two points of light, A and B, shine through the hole in the screen S and produce two dots of light, a and b, on the surface T. The two pencils of light do not practically interfere with each other although they pass through the same small hole, nor would any greater number; so that an illuminated object, which may be regarded as consisting of an infinite number of points of light, would give an image on the surface T. The disadvantages of a small hole, or "pinhole," for the production of images are (1) it must be so small that it lets very little light through and therefore gives a very feeble image, (2) that it can never give a sharp image. The first disadvantage is obvious. With regard to the second, a little consideration will show that the image of a point must be larger than the hole itself, it is always larger though it may have a central brighter part that is smaller. If the hole is reduced in size beyond a certain limit, it gives an increased spreading of light on the surface, so that a sharp image can never be produced.

Fig. 2.

Now the function of a lens is to obviate these drawbacks as far as possible; namely, to let more light through and form a brighter image, and to give sharper definition. In figure 2, the lens L collects all the light that falls upon it from the point B, and condenses it to the point b on the surface T. The light from the point A that falls on the lens is also condensed and would be brought to a point or "focus" at a beyond the surface T, but on the surface the light forms a patch of considerable size. Suppose that the lens is thirty times the diameter of the pinhole its area is 900 times as large, and the light that falls upon it is 900 times as much as the light that passes through the hole. Such an enormous gain of light is worth so much that photographers willingly put up with the very many imperfections of lenses for the sake of it, and if to this gain there is added the superior definition that is possible, it will be seen that lenses are indispensable to the photographer. To take a Daguerreotype portrait with a pinhole might have required several days if not weeks exposure of the plate and therefore would have been impossible, so that the gain in brightness of image is a great deal more than a mere convenience.

It will be observed in figure 1 that both points of light, A and B produce images on the surface T, although they are at different distances from it, but in fig. 2, although the effect of the lens is to concentrate the light from both points to two other points, one of these is beyond the surface T. This is a disadvantage inherent in lenses. They have so many other imperfections or "aberrations" that it is desirable to consider these separately. The reader should bear in mind that the one aim of opticians in perfecting lenses is to concentrate as much light as possible from each point in the object to a corresponding point, or as small as possible a dot, in the image, and the image should be flat because the plates used in photography are flat.

Fig. 3.

Spherical Aberration.—The surfaces of lenses are always ground to spherical curves, and this fact makes it impossible for a single lens, such as that shown in figure 2, to bring to a point all the light that falls upon it from a point. If a pencil of light passes through a piece of glass with sloping sides it is bent or "refracted" towards the thicker part of the glass, and the greater the angle of inclination of the two sides the more is it refracted from its original path. In figure 3 it is clear that the two sides of the lens shown in section are inclined to each other at a continually increasing angle as they approach each other at the edges of the lens. The refracting effect of the lens increases from the centre outwards, and it increases to a greater extent than is necessary to bring the incident light to a point. The focus of the pencils of light that pass through the edges of the lens is nearer to the lens than the focus of the pencils that pass through its central part. In the figure two foci are shown, a and b, but of course, in fact, intermediate parts of the lens produce intermediate foci, and what should be a point in the image, is spread out into a line on the axis of the lens, and all along this line is surrounded with the light that either is coming to a focus or that has come to a focus and has spread out again. On a screen placed at b there would be a point of light surrounded by a halo, while at a, nearer the lens, the central focus or point is surrounded by a brighter or more condensed light, and the appearance is of a circular patch of light with a brighter boundary. This is positive spherical aberration. Negative spherical aberration is due to over correction, the focus of the light passing through the margins being furthest from the lens, and the appearances on a screen are of course reversed.

Chromatic Aberration.—When light is refracted, that is bent out of its original path by a single piece of glass, it is not refracted as a whole, but each constituent behaves as if none other were present. Ordinary white light or daylight is a mixture of many coloured lights as seen in the rainbow, and when refracted, the blue is bent more than the green, the green more than the yellow, and the yellow more than the red. So that using a single lens the focus of the blue light is nearer the lens than the focus of the red light and the others come in between. In figure 4 this is represented in an exaggerated degree to make it more distinct. It will be observed that a screen placed at the focus of the blue light will show a reddish margin and if removed further from the lens the margin or halo will be bluish.

Fig. 4.

These two aberrations, spherical and chromatic were the principal faults that opticians had to deal with, because they affect the whole of the image, even the very central parts. But in photography it is necessary to get an image of a very large size as compared with the focal length of the lens, and there are some faults that only begin to show themselves at a little distance from the centre of the image and increase as the distance from the centre is greater. These aberrations were, practically speaking, incurable until a few years ago, but as recent optical advances have provided kinds of glass by the use of which they may be eliminated, or nearly so, they have become of practical importance. They are astigmatism and curvature of field.

Astigmatism and Curvature of Field.—If a diagram of suitable size is made with a series of concentric circles and radial lines upon it, and the centre of it is arranged exactly opposite the centre of the lens, and in a line with the centre of the focussing screen, the screen and diagram being parallel, then if the lens suffers from astigmatism it will be found impossible to get the outer circles and the radial lines where they cross them simultaneously focussed. Where this difficulty begins the astigmatism begins, and the greater the difference there is between the focal planes of the radial lines and the circles, the greater is the astigmatism. It will probably be found with any of the older types of lenses that neither is in focus at the same time that the centre of the diagram is, but that the screen must be racked in; this is due to curvature of field, and the difference between the curvature of field for the circles and the radial lines is due to astigmatism. In the older lenses a flatter field could only be obtained by the introduction of astigmatism, but now by the employment of the new glasses made at Jena, it is possible to practically eliminate astigmatism, and still keep the field flat.

Fig. 5.

The Development of Photographic Lenses.—When photography was first practised the best lenses available were those made for use as telescope objectives, and they had to be used with a small diaphragm to get good definition over a sufficient field. With the slow processes then in vogue a more rapid lens was much desired, and Voigtlander introduced a "portrait" lens constructed according to the results of the calculations of Professor Petzval. This portrait lens is still very largely used, and figure 5 will serve to show its general character and will be a guide to the putting of one together correctly if it has been taken to pieces for cleaning. A rapid lens such as this could not cover a sufficiently large field for landscape work, so that single lenses were still used for work in which rapidity was not of very great importance. Single lenses were improved, and other kinds of lenses were introduced from time to time, but it was not till 1866 that the "rapid rectilinears" or "rapid aplanats," called later "rapid symmetricals," and by innumerable other names according to the fancies of the makers, were introduced. Probably no lens has been made in such large numbers as this.

At about the same time, Dallmeyer introduced his portrait lens in which the position of the convex and concave elements of the back combination is reversed, the concave lens being outside, and this gives the photographer the opportunity of screwing it back a little, and so introducing a measurable amount of spherical aberration which has the effect of modifying the otherwise exceedingly fine definition at the centre of the field, and giving a greater depth of definition.

In 1881, Messrs. Abbe & Schott began a series of experiments in the manufacture of optical glasses, and they were so successful in making new and useful varieties, that an optical glass factory was eventually established at Jena, by Schott & Co. By the use of these newer glasses the limitations that had previously restricted opticians were removed, and it became possible to correct astigmatism and secure a flat field at the same time. Zeiss of Jena, towards the end of 1890, introduced his first series of "anastigmats." The "concentric" lens of Ross was introduced in 1892, a lens which probably remains unsurpassed for flatness of field and freedom from astigmatism; but as spherical aberration is present to a notable degree, an aperture of about f/22 is the largest that gives sharp definition. The "double anastigmat" of Goerz of Berlin was put on the market in 1893. It is a symmetrical lens, and in this different from the Zeiss anastigmats that preceded it. It consists of two similar combinations, each of three lenses cemented together. The unsurpassed qualities of this lens stimulated other opticians to seek to rival it, and there appeared similar lenses with four and even five lenses in each combination, besides other lenses that are more or less a copy of the double anastigmat. One of the most notable of these is the "satz-anastigmat" of Zeiss, each combination consisting of four lenses cemented together and forming an excellent single lens. These combinations are interchangeable in the same mount so that with, for example, one mount and three lenses, six different focal lengths can be obtained, as the lenses may be used singly or any two together as a doublet.

The "Cooke" lens is remarkable for the simple means by which the various corrections are made, consisting as it does of only three single lenses separated from each other. Obviously it must be used entire. These lenses do not cover so large a plate in proportion to their focal lengths as most of the other anastigmats, but perform excellently over the plates for which they are constructed.

The "stigmatic" of Dallmeyer is the latest lens of general utility. It gives good definition to the margin of the circle of light that it transmits, reduction of aperture being necessary, when its full field is employed, to get equality of illumination rather than to improve the marginal definition. Its two combinations are different, and either may be used alone as a single lens, giving focal lengths of approximately one-and-a-half and twice the focal length of the whole lens.

The "planar" of Zeiss introduced just as we write, is a symmetrical doublet characterized by a very large aperture, from f/3.6 to f/4 up to 10 inches in focal length, and a little smaller above that. It is therefore comparable with portrait lenses. Although it is symmetrical, a single combination cannot with advantage be used alone as a single lens. Telephotographic lenses are subsequently referred to.

The one aim of opticians in improving photographic lenses has been to get good definition all over a comparatively large flat surface without having to use small apertures. A defining power on the axis of the lens, that is, at the centre of the field, far exceeding what can be taken practical advantage of in ordinary photography, has long been possible. But until recently, the defining power always rapidly deteriorated as the distance from the centre was increased. But to judge of the quality of a lens, or to compare one lens with another, there are other matters that must be understood, and these we shall proceed to consider. Focal length, aperture and image angle are the chief details concerning lenses, granting that the aberrations referred to above are satisfactorily corrected.

Focal length.—The focal length or focal distance of a thin lens is the distance between it and the point to which it converges parallel rays. The rays of light are parallel when they issue from an object at an infinite distance. For ordinary practical purposes, any object, that is not nearer than a thousand focal lengths of the lens may be regarded as at an infinite distance, that is the image of an object so far off, and the image of the sun or stars (which are situated at the nearest approach to an infinitely great distance that we know of) would if separately focussed give an inappreciably small difference of position of the focussing screen. But no photographic lens is very thin. The measurement from the back surface of the lens to the screen, when focussed on a distant object, is called the "back focus," but this is of no use whatever except as to the determining of the camera length necessary. The "equivalent focal length" is the focal length (or focal distance) of a thin lens that would give the same effect, so far as focal length is concerned, as the lens in question. When the simple expression "focal length" is used, it always refers to the equivalent focal length. The single word "focus" is sometimes used erroneously instead of "focal length."

The focal length of all lenses (except to a very small extent, with single or so-called "landscape" lenses) is proportional to the linear dimension of the image that it gives under similar conditions. For example, a lens of 6 inches focal length will give just the same amount of subject on a quarter plate that a lens of 12 inches focal length will give on a whole plate, because the linear measurement of the whole plate is exactly double that of the quarter plate. The easiest way to compare the focal lengths of two lenses, is to focus both on a fairly distant object or view, and to measure in the image the distance between two fixed points in both cases. The proportion between these measurements is the proportion between the focal lengths of the lenses. By this method the focal length of any lens can easily be determined if one has a lens of known focal length.

If a lens is first focussed on a distant object, and the focussing screen is then moved back until the image of any object is of the same size as the object, the distance travelled by the focussing screen is exactly the focal length of the lens. It is however exceedingly difficult to get at the same time an image of an exactly predetermined size, and to secure the very best definition, so that it is more convenient to get the image as near as it happens to come to the size of the object and then to allow for the difference, as then nothing interferes with the operation of focussing. The best near object to use is an accurately divided scale, and the details wanted in addition to those mentioned above are the comparative lengths of the image and the object. To get these, two fine marks are made on the focussing screen, and the distance between these is the length of the image. The scale is focussed with critical exactness and so that it falls over these marks, then the amount of the scale represented between the marks can be measured, and the divisions counted for the length of the object. The distance over which the focussing screen was moved between the two focussings is to be multiplied by the length of the object and divided by the length of the image, and the result is the focal length of the lens.

Aperture.—The "aperture" of a lens is the diameter of the cylinder of light that it can receive and transmit. If the diaphragm is in front of the lens, the hole in the diaphragm is the aperture, but if the diaphragm is behind a part of the lens, so that the incident light passes through a lens first, the hole in the diaphragm is not the "aperture," the "aperture" is larger because the lens condenses the light before it gets to the diaphragm. The aperture of any lens can be measured by focussing a distant object, then replacing the focussing screen by a sheet of cardboard with a pinhole in the middle of it. In a dark-room a light must be placed behind the pinhole, and a bit of ground glass held in front of the lens. A disc of light will be seen on the ground glass and the diameter of this is the diameter of the aperture, or simply, the "aperture," with the diaphragm employed.

Rapidity.—The rapidity of a lens depends almost wholly on its focal length and aperture. The thickness of the glass makes a little difference, and at every surface in contact with air there is loss by reflection, but these and analogous matters are of comparatively little importance, and as they are uncertain and cannot be determined it is customary to refer rapidity to the focal length and aperture only. The aperture found, that is, the diameter of the effective incident cylinder of parallel rays, should be divided into the focal length, and the diaphragm corresponding to the aperture should then be marked with a fractional expression indicating the proportion of aperture to focal length. Thus if the aperture is one eighth the focal length, it is marked f/8, if a sixteenth f/16, and so on. All lenses with the same aperture as so marked may be regarded as of equal rapidity whatever their focal lengths may be. Now the more rapid a lens is the shorter the exposure that it is necessary to give for any subject, and the exposure required is proportional to the square of the figure in the expressions as given above. Namely 8 and 16 squared give 64 and 256 which are as one to four, the proportional exposures required. Or we may say that 8 to 16 are as 1 to 2 and square these and get 1 to 4 the proportional exposures.

Fig. 6.

The best way to mark stops is, for example, f/8 and f/16, as these expressions are universally understood, but some persons think that the relative rapidities or intensities are better, others prefer to express the relative exposure necessary, and every system of numbering on these plans has a unit which is merely empirical, not one of them adopting the only true or scientific unit of f/1.

Zeiss has recently changed his unit from f/100 to f/50. Dallmeyer marks some of his lenses now with the practical expression. The following table may be of service to those who happen to have lenses with their diaphragms marked on any of these empirical systems.

Image Angle.—The image angle represents what is called covering power. It may be expressed in terms of the focal length, and doubtless this is the best method, but it is not customary. It may be expressed as an angle, the angle formed when a line is drawn from each extremity of a line equal to the diameter of the circle covered, and caused to meet at a point distant from the base line equal to the focal length of the lens. The angle where the two lines meet is the image angle. But generally the covering power is expressed more roughly, as the ordinary size of the plate that sufficiently good definition can be obtained on.

Tele-Photographic Lenses.—If a negative (or dispersing or concave) lens is introduced between the ordinary lens and the plate, the equivalent focal length of the arrangement is greater than that of the ordinary lens alone, but the length of camera necessary is not proportionately great. It is possible therefore to obtain an image of a size that would otherwise require a lens of long focal length and a corresponding and perhaps impossible length of camera. But this is not the only advantage, for if the ordinary lens and the negative lens are separable to a variable extent, the amount of magnification of the image, or increase in the equivalent focal length of the optical system, is adjustable at will. For further details concerning tele-photographic lenses and their use, reference should be made to Mr. Dallmeyer's pamphlet on the subject.

There are two other subjects connected with the production of images by photographic lenses that must be referred to, though neither of them is of great importance if we exclude the use of hand cameras (which are separately treated of) and bear in mind the ordinary practice of to-day. These are depth of definition and the distortion due to the use of single lenses.

Depth of Definition.—It has already been shown that the action of the lens is to bring to a point in the image all the light that falls upon it from the corresponding point of the object. Now it is clear from fig. 2 that, if different parts of the object are at different distances from the lens, and this must be the case with solid objects, these different parts cannot be in focus at the same time. Still it is possible to get them so nearly in focus that the result is serviceable, and the ordinary method of doing this is to examine the image on the ground glass, and if the whole subject is not sharp enough, to reduce the size of the aperture. Depth of definition is increased by using a lens of shorter focal length or by reducing the aperture. If a large aperture has to be used, the focal length must be short if much depth of definition is wanted, or conversely, if the focal length must be long the aperture must be small. It follows that very rapid lenses that have a very long focus are of no use, for in portraiture, for example, this combination of properties would lead to the ear in the image being fuzzy if the eye was sharp.

If a lens were perfect and had a flat field, the depth of definition would depend only on the aperture and focal length. But if the lens gives inferior definition towards the edges of the field, it is quite obvious that there must be less depth of definition there, if a minimum of defining power is accepted. The definition at its best may be inferior to the minimum accepted and then obviously there is no depth. Depth of definition therefore at the centre of the plate depends entirely on the focal length and aperture, but away from the centre it depends also on the quality of the lens, and is much greater in a flat field anastigmat than in a lens of an older type. But depth of definition is not a quality apart, it depends entirely upon other factors, and it is better in examining a lens to determine these factors separately rather than to lump them together as depth.

Distortion produced by single lenses is due to the fact that the diaphragm is either in front of or behind them. If the diaphragm is in front, the image is drawn towards the centre of the plate to an extent that increases as the margin of the field is approached. A line along one side of the plate has its ends drawn in to a greater amount than its centre, because they are further from the middle of the plate, and therefore it becomes curved like the side of a barrel, and this effect is called barrel-shaped distortion. If the diaphragm is behind the lens, the displacement is outwards, also increasing towards the edges of the field, and a straight line at the edge of the plate becomes curved so that it is convex towards the centre of the plate. This is known as hour-glass distortion. Both these effects are illustrated (and exaggerated for clearness' sake) in fig. 6, the central square representing the true figure. This "curvilinear distortion" is absent in all cases in the middle of the plate and generally for a considerable area, and if single lenses of only long focal length are used, say of a focal length equal to at least one and a half times the length of the largest side of the plate, it may be neglected. Wide-angle single lenses should never be used except on a suitably small plate, so that the above conditions hold. The nearer the diaphragm is to the lens the less is the distortion, and some of the most modern single lenses have the diaphragm so near that the photographer is even more safe in the use of them.

MELTON MEADOWS.
A. HORSLEY HINTON.

The Comparison and Use of Lenses.—The optician when he tests lenses looks for each fault individually, but this the ordinary photographer is hardly able to do, nor is it particularly desirable for him, because if a lens is inferior it matters little to him why it is so. On the other hand occasion may arise when he wants to identify a fault, then the information already given will probably be sufficient to enable him to do so, if to it is added that a small pinhole with a flame behind it is a convenient point of light, and that if the image of this luminous point is examined with a good eyepiece, without the focussing screen, at various parts of the field, the character of the defect may be discovered.

The main things that the photographer needs to look to in judging of a lens or comparing it with another, are (1) that it works to focus, (2) the quality of its defining power especially towards the edges of the plate. There must also be taken into account the focal length and aperture, and if both these are not the same in the lenses to be compared they should be nearly the same, and the proportion that the aperture bears to the focal length should be exactly the same. A special diaphragm may have to be cut out of card for one of them. The best test object that is always at hand is a newspaper pinned flat against a flat wall. The camera must not be moved during the work. Each lens is very carefully focussed and a negative made, using the same aperture, time of development, and in all ways similar treatment for both. If the focal lengths are different, the images will be of correspondingly different sizes, and then the same detail must be compared, not the definition at the same distance from the centre.

All good lenses work to focus, but some of the cheaper ones do not. To test this, any series of small objects arranged side by side, but at distances varying by intervals of say two inches from the camera, is photographed after carefully focussing on the middle one. If any other than the middle one is the best defined, the lens is at fault. But in this, as in all similar tests, it must be remembered that ordinary dry plates are not quite flat, and the error of the plate may make an appreciable difference.

The use of lenses comprises the whole art of working with the camera, it is therefore not our province to say much about it. But so far as lenses themselves are concerned it may be remarked that, if a lens has a round field, it may be advantageous to tip up the lens with regard to the plate when only a part of the plate is being used, as for example sometimes in taking a landscape. But in using the modern flat field lenses special care should be taken to keep the lens and plate exactly true to each other, the plate exactly at right angles to the lens axis. The image and plate must coincide or definition will suffer. If the image is rounded and the plate flat, then in any case the result is only a compromise, but to take full advantage of the larger apertures when the field is flat, much more care than has been usual must be devoted to this matter.

Simple uncorrected lenses such as spectacle lenses or "monocles," suffer from the defects that have already been described, and are valued on this account by some workers because they give blurred or "soft" images. With a small enough diaphragm they will give good definition, and generally it may be stated that reducing the aperture lessens the effect of any fault that a lens may possess. To get the best definition that a simple lens will give, the plate must be brought nearer the lens after focussing by about one-fiftieth of the focal length of the lens, so that it may be brought from the best focus of visual light into the best focus of the photographically active light. If the object photographed is nearer to the lens than about one hundred times its focal length, the amount of movement after focussing must be increased. If four focal lengths distant, the correction is nearly one-thirtieth of the focal length, at three focal lengths distant, nearly one-twentieth, and at two focal lengths, about a thirteenth.

Pinholes give an image that for all practical purposes may be said to be equally blurred or "soft" over the whole plate. Much has been written about pinholes and their use, but it is not definitely known yet whether the exposure should be longer or shorter than the exposure required when a lens is used, allowing, of course, for the smallness of the aperture. The following short table and exposure rules from the writer's "Science and Practice of Photography," will probably prove useful:—

All the above figures are in inches. Whatever pinhole and at whatever distance, estimate the exposure for a lens at f/16, f/22, f/32, f/45, or f/64, as the case may be, and multiply it by the square of the number of inches that the plate is distant from the pinhole. But if the distance is as given above for any hole, it is sufficient to expose for as many minutes as the plate is inches distant from the hole, for a subject that would require one second with an aperture of f/16.

Chapman Jones.

Portraiture.