Ski-optometer Master Model 215

Embodying in a Single Instrument, in Convenient Form,
Cylindrical and Spherical Lenses, in Combination
with Appliances for Testing and Correcting
Muscular Imbalance.

Refraction and
Muscular Imbalance

As Simplified Through the Use
of the Ski-optometer

By

DANIEL WOOLF

WOOLF INSTRUMENT CORPORATION
New York: 516 Fifth Avenue

Copyright 1921
By WOOLF INSTRUMENT CORPORATION

Published by
Theodore S. Holbrook
New York

CONTENTS

The demands of the day for maximum efficiency in the refracting world are largely accountable for the inception, continuous improvement and ultimate development of the master model Ski-optometer.

The present volume, dealing with the instrument’s distinctive operative features, has been prepared not only for Ski-optometer users, but also for those interested in the simplification of refraction and muscular imbalance.

The author is indebted for invaluable counsel, to

Louis J. Ameno, M.D., New York.
E. LeRoy Ryer, O.D., New York.
Jos. D. Heitger, M.D., Louisville, Ky.
W. B. Needles, N.D., Kansas City, Mo.

INTRODUCTORY

While in a measure the conventional trial-case still serves its purpose, so much of the refractionist’s time is consumed through the mechanical process of individually transferring the trial-case spheres and cylinder lenses, that far too little thought is given to muscular imbalance, notwithstanding its importance in all refraction cases.

Dr. Samuel Theibold, of Johns Hopkins University, in a recent address before the American Medical Association, stated that the average refractionist was inclined to devote an excess of time to general refraction, completely overlooking the important test and correction of muscular imbalance. If the latter is to be at all considered, general refraction must be simplified—without impairing its accuracy—a result that is greatly facilitated through the use of the Ski-optometer.

One must admit that tediously selecting the required trial-case lens—whether sphere, cylinder or prism—watching the stamped number on the handle—continual wiping and inserting each individual lens in a trial-frame is a time-consuming practise. This is readily overcome, however, through the employment of the Ski-optometer.

In a word, the Ski-optometer is practically an automatic trial-case, bearing the same relation to the refracting room as the accepted labor and time-saving devices of the day bear to the commercial world.

The present volume has accordingly been published, not alone in the interest of those possessing a Ski-optometer, but also for those interested in attaining the highest point of efficiency in the work of refraction and muscular imbalance.

Ski-optometer Lens Battery (almost actual size)
showing how sphere and cylinder lenses are procured.

After obtaining FINAL results, your prescription is automatically registered,
ALL READY for you to transcribe.

Fig. 1—The three time-saving moves necessary in the operation of the Ski-optometer.

Chapter I
SKI-OPTOMETER CONSTRUCTION

A far better understanding of the instrument will be secured if the refractionist possessing a Ski-optometer will place it before him, working out each operation and experiment step by step in its proper routine.

The three moves as outlined in [Fig. 1] should first be thoughtfully studied and the method of obtaining the spheres and cylinders carefully observed.

Fig. 2—To Obtain Plano.

The instrument should then be set at zero or “plano,” a position indicated by the appearance of the three “0 0 0” at the spherical register, in conjunction with one “0” or zero, for the cylinder at its register, marked “CC Cyl.”

After this move, the supplementary disk’s pointer should be set at “open” ([Fig. 2]).

Fig. 3—To obtain sphericals, turn this Single Reel as shown by dotted finger. This assures an automatic and simultaneous registration at sphere indicator of focus of lens appearing at sight opening.

Convex Spherical Lenses

A careful study will show that the Ski-optometer’s spherical lenses are obtained by merely turning the smaller reel ([Fig. 3]). The first outward turn of this reel, toward the temporal side of the instrument, draws into position in regular order the spherical lenses +.25, +.50, +.75, and +1.D., as shown in [Fig. 3a].

3-A—Outer spherical reel containing Cx. sphericals
from .025 to 1.00D and a blank.

3-B—Inner spherical disk containing Cx. sphericals,
automatically turns within 3-A.

3-C—Supplementary spherical disk.

By means of a concealed tooth gear, an inner disk is automatically picked up, placing its first lens +1.25D in position ([Fig. 3b]). This +1.25D spherical lens remains stationary while the outer disk again revolves, adding to it the original +.25, +.50, +.75 and +1.D., the latter totalling +2.25D. At this point, the instrument again automatically picks up its inner disk, thereby placing its second lens, +2.50D, in position.

Fig. 4—With the reappearance of “00” at sphere indicator, a rapid increase or decrease of +1.25 is accurately and speedily attained.

Instead of using intermediate strengths in making an examination, it is frequently desirable to make such extended changes as 1.25D to 2.50D. With the Ski-optometer, the refractionist will note that two white zeros appeared at the spherical register in connection with +1.25, and again with +2.50. A rapid outward turn of the spherical reel toward the temporal side to the point of the reappearance of the two zeros will show +3.75D; or, if increased power is still desired, a rapid turn will draw +5.D. into position ([Fig. 4]).

Turning the reel inward toward the nasal side will likewise decrease its convex power. In brief, each one of these lenses, showing their foci in conjunction with the two white zeros, are signals indicating the rapid increase or decrease of one and one-quarter diopter. After continuing to +6D., the next turn automatically shows zero (or “plano”), the original starting point, which is again indicated by the three white zeros.

Through the turn of the single reel—an exclusive Ski-optometer feature—all convex spherical lenses have now been attained in quarters up to +6.D, practically covering ninety percent of all refraction cases.

Fig. 5—With supplementary disk pointer set at +6 Sph., this places an additional +6.D spherical lens at sight opening, extending instrument’s total convex spherical power to +12.D.

Should still greater power be desired, the small pointer at the outer edge of the instrument should be set at +6 sphere ([Fig. 5]). This controls a supplementary disk ([Fig. 3c]) which places an additional +6D. lens before the original range of lenses previously referred to, thus increasing the maximum power to +12D. If still greater strength is required, any additional trial-case lens may be added, a cell being provided for that purpose on the forward plate of the instrument.

Operates and Indicates Automatically

As previously explained, in using the Ski-optometer, it is only necessary to remember that each outward turn of the single reel toward the temporal side of the patient increases the plus power, while the reverse turn toward the patient’s nose decreases it. In fact, no attention need ever be given the register until the required sum-total is secured, it only being necessary to turn the single reel in order to be assured of the unvarying and accurate operation of the instrument.

For convenience, the contour or upper edge of the plate covering the spherical reel has been made to fit the index finger ([Fig. 3]). Hence the operator should note that it requires but one complete turn from extreme side to side, rather than a number of short turns, in order to bring each individual lens into position, thus obtaining the full advantage of the automatic spring-stop. This likewise permits the refractionist to operate the Ski-optometer even though the room is in total darkness.

Concave Spherical Lenses

Another simple and exclusive Ski-optometer advantage worthy of note is the method employed in obtaining concave, spherical lenses. Instead of employing a battery of concave lenses similar to the convex battery previously described, the instrument’s operation is greatly simplified through the use of a neutralizing process.

In short, the Ski-optometer only contains two concave lenses to obtain its entire series—namely, a -6.D and a -12.D sphere ([Fig. 3c])—first setting the pointer of the supplementary disk at -6. sphere, then setting the indicator of the spherical battery at +6.

Thus zero (or plano) is obtained, the plus neutralizing the minus.

By merely turning the plus or convex spherical reel inward, or toward the patient’s nose, the convex power is then decreased, naturally increasing the concave value or total minus lens power. For example, if the spherical indicator shows +5.D, when the -6D. lens is placed behind it, the lens value at the sight opening will be -1D ([Fig. 6]). If required, the refractionist may continue on this plan until only the -6D. lens remains.

Fig. 6—With this indicator of supplementary disk, set at -6.D. Sph. and spherical indicator at +5.D—lens value at sight opening is -1.D. Sph. This simple arrangement makes it possible to operate the Ski-optometer with but Single Reel for both plus and minus sphericals.

Should concave power stronger than -6D. be desired, by placing the pointer of the supplementary disk at -12D. Sph. and proceeding to neutralize as before, all the concave powers up to -12D. in quarters are similarly obtained. For the convenience of the operator, all minus or concave spherical powers are indicated in red; while plus, or convex powers, are indicated in white.

The instrument is also provided with an opaque or blank disk which is brought into position before the sight opening by setting the pointer of the supplementary disk at “shut” ([Fig. 3c].)

Summing up, all plus and minus spherical powers have been attained from zero to 12D. in quarters, practically through the turn of the single reel—a simplicity of operation largely responsible for Ski-optometer supremacy.

Chapter II
CYLINDRICAL LENSES

It is commonly admitted that setting each trial-case cylindrical lens at a common axis is the most tedious part of refraction.

The automatic cylinder, one of the Ski-optometer’s latest and distinctly exclusive features, not only overcomes this annoyance but also avoids the need of individually transferring each cylindrical lens according to the varying strengths.

Fig. 7—Once you set the axis indicator as shown by dotted fingers, each cylindrical lens in the instrument automatically positions itself exactly at that axis, as indicated by the arrow.

By merely setting the Ski-optometer’s axis indicator ([Fig. 7]), each cylindrical lens in the instrument automatically positions itself, so that it will appear at the opening at the exact axis indicated.

This is readily accomplished by placing the thumb on the small knob, or handle of the axis indicator, drawing it outward so as to release it from spring tension. The indicator may then be set at any desired axis; and, on releasing the handle, every cylinder in the instrument becomes locked, making it impossible for any lens to appear at an axis other than the one specified by the indicator.

This insures the absolute accuracy of the axis of every cylinder as it appears before the patient’s eye. Subsequent shifting of the axis even to a single degree is impossible, although it is a common occurrence where trial-case lenses are employed.

Obtaining Correct Focus

After setting the axis indicator, the only remaining move is to obtain the correct cylindrical strength or focus. This is readily accomplished by merely turning the Ski-optometer’s larger or extreme outer single reel, which contains concave cylindrical lenses from .25D to 2D in quarters ([Fig. 8a]). It should again be borne in mind that a downward turn increases concave cylinder power, while an upward turn decreases it. The operation of the cylinder reel is greatly facilitated by carefully noting position of thumb and index finger ([Fig. 8]). Thus accuracy of result, simplicity of operation and the saving of much valuable time is invariably assured.

Fig. 8A—Inner cog-wheel construction, showing arrangement of Ski-optometer cylinders. This simple construction assures accuracy and avoidance of the slightest shifting of axes.

As each cylinder appears before the patient’s eye, it simultaneously registers its focus at the indicator marked “CC CYL” shown in [Fig. 8]. Examinations of greater accuracy could not possibly be made than those obtained through the Ski-optometer, hence no refractionist should hesitate to employ it throughout an entire examination—wherever trial-case lenses are used.

The range of the Ski-optometer’s cylinder lens battery includes up to 2D. in quarters. An axis scale and a cell is located at the back of the instrument for insertion of an additional trial-case cylinder lens, when stronger cylindrical power is required. For example, if an additional -2D. cylinder is added, it will increase the range up to 4D. cylinder; or if twelfths are desired, a 0.12D. cylinder lens may be inserted. In this connection, it is interesting to note that considerable experimenting with twelfths in the Ski-optometer proved them to be needless, inasmuch as the instrument’s cylindrical lenses set directly next to the patient’s eyes overcome all possible loss of refraction, as explained in a later paragraph.

Fig. 8—Turn this Single Reel as shown by dotted finger to obtain cylindrical lenses, which simultaneously register their focus as they appear. Each lens also automatically positions itself at axis designated.

Why Concave Cylinders Are Used Exclusively

The Ski-optometer contains only concave cylinders, as it is universally admitted that convex cylinders are not essential for testing purposes.

In fact, concave cylinders should alone be used in making an examination, even where a complete trial-case is employed. To repeat one of the first rules of refraction: “As much plus or as little minus spherical power as patients will accept, combined with weakest minus cylinder, simplifies the work of refraction and insures accuracy without time-waste.”

After an examination with the Ski-optometer is completed, the total result of plus sphere and minus cylinder may be transposed if desired, though in most instances it is preferable to prescribe the exact findings indicated by the instrument. This will also avoid every possibility of error, eliminating responsibility where one is not familiar with transposition—since, after all, it is the duty of the optician to thoroughly understand that part of the work.

Transposition of Lenses

It is commonly understood that transposition of lenses is merely change of form, but not of value.

For example, a lens +1.00 sph. = -.50 cyl. axis 180° may be transposed to its equivalent, which is +.50 sph. = +.50 cyl. axis 90°. The accepted formula in this special instance is as follows: Algebraically add the two quantities for the new sphere, retain the power of the original cylinder, but change its sign and reverse its axis 90 degrees. Applying this rule, a lens +.75 sph. = -.25 cyl. axis 180°, is equivalent to +.50 sph. = +.25 cyl. axis 90°.

Similarly, a lens +1.00 sph. = -1.00 cyl. axis 180° is equivalent to +1.00 cyl. axis 90°.

One of the difficulties in transposing is in reversing the axis. In such cases, it is well to memorize the following simple rule:

To reverse the axis of any cylindrical lens containing three numerals—add the first two together and carry the last. For example, from 105 to 180 degrees, etc.:

In using the Ski-optometer, it is absolutely unnecessary to transpose the final result of an examination; merely write the prescription as instrument indicates. The idea that plus sphere combined with minus cylinder, or the reverse, is an incorrect method of writing a prescription, has long since been disproved.

Chapter III
HOW THE SKI-OPTOMETER ASSISTS
IN REFRACTION

The construction of the Ski-optometer has now been fully explained, and the reader realizes that since the instrument contains all the lenses necessary in making an examination, greater operative facility is afforded through its use than where the trial-case lenses are employed.

The Ski-optometer is “an automatic trial-case” in the broadest sense of the term, wholly superseding the conventional trial-case. It should therefore be employed throughout an entire examination, wherever trial-case lenses were formerly used. To fully realize its labor saving value in obtaining accurate examination results, it is only necessary to recall the tedious method of individually handling and transferring each lens from the trial-case to the trial-frame, watching the stamped number on each lens handle, wiping each lens and in the case of cylindrical lenses setting each one at a designated axis—all being needless steps where the Ski-optometer is employed.

The Use of the Ski-optometer in Skioscopy

In skioscopy, the Ski-optometer offers the refractionist assistance of the most valuable character.

For example, assuming that extreme motion in the opposite direction with plane or concave mirror is obtained with a +1.25D. spherical lens before the patient’s eye; by quickly turning the Ski-optometer’s single reel until the two white zeros again appear, +2.50D is secured, as explained in the previous chapter. If this continues to give too much “against motion,” the lens power should be quickly increased to +3.75 or +5.00D if necessary ([Fig. 4]). Should the latter reveal a shadow in the reversed direction, the refractionist is assured that it is the weakest lens that will cause its neutralization. Practically but few lenses have been used to obtain the final result proving the instrument’s importance and time-saving value in skioscopy, and demonstrating the simplicity with which tedious transference of trial-case lenses is avoided.

Furthermore, it should be noted that where the Ski-optometer is used in skioscopy, it is not necessary to remove the retinoscope from the eye or to constantly locate a new reflex with each lens change. This permits a direct comparison of the final lens and eliminates the usual difficulty in mastering skioscopy. The chief cause of this difficulty is due to the fact that the transferring of the trial-case lenses makes it practically impossible for the student to determine whether the previous lens caused more “with” or “against” motion.

Fig. 9—The Woolf ophthalmic bracket. A convenient and portable accessory in skioscopy and muscle testing; can be used with or without Greek cross.

Where the indirect method is employed in skioscopy, best results are secured through the use of the Woolf ophthalmic bracket and concentrated filament lamp, together with an iris diaphragm chimney. The latter permits the reduction or increase of the amount of light entering the eye, as it is agreed that a large pupil requires less light, a small pupil requiring more light. The bracket referred to permits the operator to swing the light into any desired position ([Fig. 9]), while the iris diaphragm chimney serves as a shutter. This apparatus may also be employed for muscle testing, as described in a subsequent paragraph.

A Simplified Skioscopic Method

In using the Ski-optometer, instead of working forty inches away from the patient in skioscopy and deducting 1.D., the refractionist will find it more convenient to work at a twenty inch distance, deducting 2.D. This working distance may be accurately measured and maintained by using the reading rod accompanying the instrument. Instead of deducting 2.D. from the total findings, however, it is preferable to insert a +2.D. trial-case lens in the rear cell of the instrument directly next to the patient’s eye. After determining the weakest lens required to neutralize the shadow in both meridians, the additional +2.D. lens should be removed and the total result of the examination read from the instrument’s register.

To illustrate a case in skioscopy where spherical lenses are employed to correct both meridians, assume that the vertical shadow requires a +1.25D lens to cause its reversal, while the horizontal requires +2.00D. Employment of the customary diagram, illustrated in [Fig. 10], would show the patient required +1.25 sph. = +.75 cyl. axis 90°, which when transposed is equivalent to +2.00 sph. = -.75 cyl. axis 180°.

Fig. 10—Where spherical lenses are employed in skioscopy, above indicates patient requires

+1.25 Sph. = +.75 Cyl. Axis 90°
or +2 Sph. = -.75 Cyl. Axis 180°

It should be noted that the total spherical power is +2.00D, as the Ski-optometer’s register shows, while the difference between the two meridians is 75, which is the required strength of the cylinder. By then turning the cylinder reel to .75, and setting the axis indicator at 180° (because by using minus cylinders, the axis must be reversed) the patient should read the test-type with ease if the skioscopic findings are correct. Thus with the Ski-optometer, it is not even necessary to learn transposition, since the instrument automatically accomplishes the work, avoiding all possibility of error.

Employing Spheres and Cylinders
in Skioscopy

Another commonly used objective method may be employed with even greater facility through the combined use of both the Ski-optometer’s spherical and cylindrical lenses. As previously suggested, insert the +2.00 spherical trial-case lens in the rear of the instrument, working at a twenty inch distance, then proceed to correct the strongest meridian first.

It was assumed that it required a +2.00 spherical to neutralize the strongest, or horizontal meridian, as shown in [Fig. 10]. The refractionist should then set the axis indicator therewith, which is the axis of the cylinder, or 180°.

It is then merely a matter of increasing the Ski-optometer’s cylindrical lens power until the reversal of the shadow in the weakest meridian is determined. Assuming this proves to be -.75 cylinder, axis 180°, the patient’s complete prescription +2.00 sph. = -.75 cyl. axis 180°, would be registered in the Ski-optometer without any further lens change other than the removal of the +2.00 working distance lens.

However, regardless of the method employed, the Ski-optometer greatly simplifies skioscopy. In fact, the instrument was originally intended to simplify retinoscopy or skioscopy, as the subject should be termed, the name “Ski-optometer” having been derived from the latter.

Use of the Ski-optometer
in Subjective Testing

In subjective refraction, especially where the “better or worse” query must be decided by the patient, it is commonly understood that the refractionist is compelled to first increase and then decrease a quarter of a diopter before the final lens is decided. With the Ski-optometer, the usual three final changes are made in far less time than it takes to make even one lens change from trial-case to trial-frame.

For example:

Assuming, with a +1.25D spherical lens before the patient’s right eye, he remarks that he “sees better” with a +1.D. while +.75D is not as satisfactory. The refractionist can then quickly return to +1.D., simply turning the Ski-optometer’s single reel outward to increase, or backward to decrease, the lens strength. So rapidly have these lens changes been made, that the patient quickly sees the difference of even a quarter diopter, and quickly replies, “better” or “worse.”

This is made possible because the eye does not “accommodate” as quickly as the lens change made with the Ski-optometer. It should also be noted that the eye receives an image on its retina within one-sixteenth of a second; otherwise, the patient is forced to accommodate, making it difficult to see the difference of even a quarter diopter. On the other hand, in transferring trial-case lenses, with its slow, tedious procedure, the patient, being unable to detect the slight difference of only a quarter diopter, unhesitatingly replies, “no difference,” merely because they are compelled to accommodate.

A Simplified Subjective Method

The following simplified method of procedure is suggested for subjective testing with the Ski-optometer, although as previously explained, the refractionist may employ his customary method, overcoming the annoyance of transferring trial-case lenses and the setting of each cylinder individually. The Ski-optometer has been constructed and based upon the golden rule of refraction: “As much plus or as little minus spherical, combined with as little minus cylinder power as the patient accepts.”

By applying this rule as in the above method and starting with +5.D. spherical, watching the two zeros ([Fig. 4]) and rapidly reducing +1.25D each time, we will assume that +1.25D gives 20/30 vision; as a final result +1.D. will possibly give 20/25 vision.

The patient’s attention should next be directed to the most visible line of type, preferably concentrating on the letter “E” or the clock dial chart—either of which will assist in determining any possible astigmatism. Since the Ski-optometer contains concave cylinders exclusively, the next move should be the setting of its axis indicator at 180°, commonly understood as “with the rule.” One should then proceed to determine the cylinder lens strength by turning the reel containing the cylindrical lenses ([Fig. 8]). Should the patient’s vision fail to improve after the -.50D. cylinder axis 180° has been employed, the refractionist, in seeking an improvement, should then slowly move the axis indicator through its entire arc.

With the cylinder added, regardless of axis, poor vision might indicate the absence of astigmatism. If astigmatism exists, vision will usually show signs of improvement at some point, indicating the approximate axis. Once the latter is ascertained, the refractionist may readily turn the Ski-optometer’s cylinder reel and obtain the correct cylinder lens strength, after which the axis indicator should be moved in either direction in order to obtain the best possible vision for the patient.

The refractionist should always aim to obtain normal (or 20/20) vision with the weakest concave cylinder, combined with the strongest plus sphere, or weakest minus sphere.

Procedure for Using Minus Cylinders
Exclusively

For the benefit of those who have never used minus cylinders exclusively in making their examinations, we will assume that the patient requires O.U. +1.D sph. = -1D cyl. axis 180° for final correction; the latter, in its transposed form, being equivalent to +1.D. cylinder axis 90°. Unquestionably the best method is the one that requires the least number of lens changes to secure the final result.

To obtain this, the following order of lens change should be made: First, +1.D. sphere is finally determined and allowed to remain in place. Concave cylinders are then employed in quarters until the final results of +1.D. spherical, combined with -1.D. cylinder axis 180° is secured. This necessitates the change of but four cylindrical lenses as shown in routine “A” as follows:

In brief the method of using minus cylinders exclusively in an examination, as explained in routine “A”, necessitates the change of the cylinder lenses only after the strongest plus sphere is secured.

On the other hand, notwithstanding innumerable other methods where plus cylinders are used, routine “B” shows that the best spherical lens strength the patient will accept, is also first determined. Then both spheres and cylinders are changed in their regular order by gradually building up in routine, by increasing plus cylinder and next decreasing sphere, a quarter diopter each time, until the final result is secured.

While it is conceded that both routine “A” and “B” are of themselves simplified methods, by comparing routine “A” where minus cylinders are used with routine “B” where plus cylinders are used in their corresponding steps, the refractionist will note by comparison that one is the exact equivalent and transposition of the other. Where plus cylinders are employed, eight lens changes are made before final results are secured; while but four lens changes are necessary where minus cylinders are used.

The refractionist should also note by comparison that the use of minus cylinders reduces focus of the plus sphere, but only in the meridian of the axis. It has not made the patient myopic. Furthermore, a plus cylinder will bring the focal rays forward, while minus cylinders throw them backward toward the retina.

This is but another reason for the exclusive use of minus cylinders in refraction.

The method of using minus cylinders exclusively in an examination, necessitates the change of the cylinder lenses only. On the other hand, the method of using plus cylinders makes it necessary to change spheres and cylinders in routine.

In brief, since using the minus cylinder is merely a matter of mathematical optics, their use even in a trial-case examination is strongly urged.

The maximum value of the Ski-optometer is fully realized only when the advantages of using minus cylinders exclusively in every examination is clearly understood.

Constant Attention Not Required

With the Ski-optometer, when the examination is completed, the sum-total of final results—whether spherical, cylinder, axis, or all combined—are automatically indicated or registered ready to write the prescription. Until then, the foci of the various lenses that may be employed are of no importance.

In short, in using the Ski-optometer, it is not necessary to constantly watch the registrations during examinations. The automatic operation of the instrument is an exclusive feature, so that the refractionist should unhesitatingly employ it. Hence, by eliminating the perpetual watch on the lenses in use, the refractionist is enabled to give his undivided attention to the patient rather than to the trial lenses.

Where a special dark-room is used for skioscopic work, an additional wall bracket or floor stand will necessitate only the removal of the instrument itself. This enables the refractionist to use the Ski-optometer for subjective or objective work, without disturbing the patient’s correction.

Chapter IV
IMPORTANT POINTS IN CONNECTION WITH
THE USE OF THE SKI-OPTOMETER

The Ski-optometer is equipped with an adjustable head-rest, permitting its lenses to be brought as close as possible to the eye without touching the patient’s lashes, a matter of importance in every examination.

Fig. 11—The nasal lines of the Ski-optometer fit the contour of face with mask-like perfection, patient remaining in comfortable position.

Elimination of Trial-Frame Discomfort

Where the Ski-optometer is correctly fitted to the face, the patient invariably remains in a comfortable position ([Fig. 11]). The instrument is shaped to fit the face like a mask, so that even with a pupillary distance of but 50 m/m (that of a child) there still remains, without pinching, ample room for the widest nose of an adult.

Before making an examination, the correct pupillary distance should always be obtained by drawing an imaginary vertical line downward through the center of each eye from the 90° point on the Ski-optometer axis scale. The pupillary distance will then register in millimeters on the scale of measurements for each eye separately. If the Ski-optometer is correctly adjusted, the patient is securely held in position, the cumbersome trial-frame being entirely eliminated.

Rigidity of Construction

Illustration on following page ([Fig. 11a]) shows the reinforced double bearing arms which hold the Ski-optometer lens batteries at two points. This eliminates possibility of the instrument getting out of alignment, and prevents wabbling or loose working parts.

The broad horizontal slides shown in the cut, move in and out independently so that the pupillary distance is obtained for each eye separately by turning the pinioned handle on either side of the instrument. The scale denotes in millimeters the P.D. from the median line of the nose outward, the total of both scales being the patient’s pupillary distance.

[Fig. 11a] also serves to show the staunch construction of the base of the Ski-optometer.

Fig. 11a—Showing staunch construction of Ski-optometer base.

How to Place the Ski-optometer in Position

The patient should be placed in a comfortable position with “chin up,” as though looking at a distant object. The instrument should then be raised or lowered by the adjustable ratchet wheel of the bracket. The wall bracket gives best results when suspended from the wall, back of the patient, as shown on [page 135]. This bracket should be placed about ten inches above the head of the average patient. When the Ski-optometer is placed in position for use, its lower edge will barely touch the patient’s cheeks. It is sometimes advisable to request the patient to lightly press toward the face the horizontal bar supporting the instrument. Particularly good results are secured where a chair with a head-rest is employed in conjunction with the Ski-optometer. (See illustration of Model Refraction Room, [Page 112]).

Cleaning the Lenses

The time-waste of perpetually cleaning lenses is overcome where the Ski-optometer is employed. For the convenience of the operator and protection of Ski-optometer lenses, the latter are concealed in a dust-proof cell, overcoming all dust and finger-print annoyances. When not in use, the instrument should be covered with the standardized hood forming part of the equipment.

The instrument should not be taken apart under any circumstances. To clean its lenses, not a single screw need be removed, as the lenses of each disk may be cleaned individually through the opening of the other disks. These openings are conveniently indicated by the white zeros ([Fig. 2]). The Ski-optometer contains but eleven spherical and eight cylindrical lenses on each side, so that the actual work of cleaning should not require over ten minutes at the most, cleaning the lenses every other week proving quite sufficient.

Accuracy Assured in Every Test

Loss of refraction is completely eliminated through the use of the Ski-optometer. The most casual examination of the trial-frame or any other instrument shows that the construction necessitates the placing of the spherical lens next to the eye with the cylinder lens outermost—a serious fault wholly overcome in the Ski-optometer.

Not only do the cylindrical lenses of the Ski-optometer set directly next to the patient’s eye, thus overcoming any possible loss of refraction, but the strong spherical lenses of the supplementary disk are set directly next to the cylinder. There is apparently but a hair’s distance between these lenses; the two disks containing the spherical lenses of the Ski-optometer likewise setting close together.

In a word, the Ski-optometer’s cylinder lenses set directly next to the patient’s eye, followed by the stronger sphericals, so that the weakest spherical or +.25 (the lens of least importance) sets farthest away. This is 3½ m/m closer than any trial-frame manufactured, however, and at least 10 m/m closer than any other instrument—another reason for implicitly relying on the Ski-optometer for uniformly accurate results.

Built to Last a Lifetime

Fig. 12—(A. and B.)—This unique, patented split-spring device of screwless construction, securely holds all movable parts. In case of repair, they may be removed with the blade of a knife.

The Ski-optometer is built on the plan of ¹/₁₀₀₀″, insuring absolute rigidity and accuracy and a lifetime of endurance. Particular and detailed attention has been given to the novel means of eliminating screws which either bind, create friction or continually work loose, causing false indications of findings on scales of measurements; hence correct and accurate indications are insured in the Ski-optometer by means of a split-spring washer construction similar to that of an automobile tire’s detachable rim ([Fig. 12]).

This patented spring washer construction securely holds the phorometer lenses, the rotary prism and the revolving cylinder lens cells.

Whenever necessary, or in case of repair, these parts may be readily removed with the blade of a knife.

Chapter V
CONDENSED PROCEDURE FOR MAKING
SPHERE AND CYLINDER TEST
WITH THE SKI-OPTOMETER

Notwithstanding various methods employed, for both subjective and objective refraction, the following synopsis of the previous chapters will unquestionably prove most valuable to the busy refractionist, enabling him to make error-proof examinations in practically every case without resorting to the transference of trial-case sphere or cylinder lenses. A careful reading of chapters one and two should be made however, so that one may gain an understanding as to how spheres and cylinders are obtained with the Ski-optometer.

Subjective Distance Test

1st—Place Ski-optometer in position, employing spirit level, thus maintaining instrument’s horizontal balance.

2nd—Adjust the pupillary distance for each eye individually, by drawing an imaginary vertical line downward through the center of each eye from the 90° point on the Ski-optometer’s axis scale. The opaque disk should be placed before the patient’s left eye by setting the supplementary disk handle at “shut.”

3rd—The Ski-optometer lens battery before the patient’s right eye should be set at “open” ([figure 2]), whereupon the first turn of spherical lens battery toward the nasal side places a +6.D sphere in position. This should blur vision of average patient.

4th—It is now only necessary to remember that an outward turn toward temporal side of the instrument increases plus sphere power, while a nasal turn decreases it. Therefore continue to reduce convex spherical lens power until the large letter “E” on the distant test card is clear. Then request patient to read as far down as possible,—a rapid turn of a quarter diopter being readily accomplished with the Ski-optometer ([Fig. 4]).

5th—In the event of working down to “zero” with spheres, the supplementary disk handle or indicator should next be set at -6.D sphere, while the spherical reel should be turned toward the nasal side—thus building up on minus spheres ([Fig. 6]). In short, the strongest plus sphere or weakest minus sphere should always be determined before employing cylinders.

6th—With the best spherical lens that the patient will accept left in place, direct attention to the letter E or F in the lowest line of type the patient can see on the distant test letter chart. Then set axis indicator at 180° ([Fig. 7]).

7th—Next increase concave cylinder power until vision is improved. If vision is not improved after increasing cylinder strength to -.50 axis 180°, merely reverse the axis to 90°. If vision is improved, cylinder lens strength should be increased. If not, it should be decreased ([Fig. 8]).

8th—Slowly move axis indicator through entire arc of axis, thus locating best possible axis ([Fig. 7]).

9th—After sphere and cylinder test of right eye has been made, place supplementary disk handle at “shut.” Then repeat procedure in testing left eye.

10th—After completing examination for each eye separately, then, with both of the patient’s eyes open, direct attention to lowest line of type he can see, concentrating on the E or F, simultaneously increasing or decreasing spherical power before both eyes. The refractionist merely recalls that by turning the Ski-optometer’s single reel toward the temporal side, convex spherical power is increased, by turning toward the nasal side for either eye, spherical power is decreased. Cylinder lens strength may be changed in a like manner before both eyes simultaneously.

11th—After making the distance test, then only is it necessary to copy the result of the examination as recorded by the Ski-optometer.

Subjective Reading Test

Tilt Ski-optometer forward in making reading test. The wide groove in the horizontal bar supporting the instrument, permits it to be slightly tilted.

12th—Place Ski-optometer reading rod in position with card at about 14 inches. Close off one eye. Direct patient’s attention to the name “Benjamin” printed at top of card.

13th—Leave cylinder lens in place. Proceed as in distance test with +6.D sphere, fogging down until the first word “laugh” on the reading card, in line 75M, is perfectly clear, this being slightly smaller than the average newspaper type.

14th—After completion of examination for each eye separately, then with both eyes direct patient’s attention to word “laugh.” Move reading card in or out a few inches either side of 14 inch mark. This will determine any possibility of an over-correction. Then record prescription just as Ski-optometer indicates. For a detailed description of above, as well as for objective testing with the Ski-optometer, read [chapter three].

Chapter VI
MUSCULAR IMBALANCE

The purpose of the present chapter is to acquaint the refractionist with the operation of the Ski-optometer as “a scientific instrument for muscle testing”—the subject being treated as briefly and comprehensively as is practicable.

As the reader progresses in the subject of muscular anomalies, he may carry his work to as high a plane as desired, increasing his professional usefulness to an enviable degree.

Through the use of the Ski-optometer, muscle testing may be accurately accomplished in less time than a description of the operation requires. Furthermore, tedious examinations may be wholly overcome through the discontinuance of the consecutive transference of the various degrees of prisms from the trial-case. In fact, the latter method has long been quite obsolete, owing to the possibility of inaccuracy. The muscle action of the eye is usually quicker than the result sought through the use of trial-case prisms; hence muscle testing with the Ski-optometer is accomplished with far greater rapidity and accuracy, thus making the instrument an invaluable appliance in every examination.

The Action of Prisms

Students in refraction—and one may still be a student after years of refracting—are sometimes puzzled as to just what a prism does when placed before an eye. They refer to every available volume and are often confused between ductions and phorias, finally dropping the subject as an unsolvable problem. In view of this fact, it is suggested that the refractionist should read the present volume with the actual instrument before him.

Before proceeding, one should first understand the effect of a prism and what it accomplishes. To determine this, close one eye, looking at some small, fixed object; at the same time, hold a ten degree prism base in before the open eye, noting displacement of the object. This will clearly show that the eye behind the prism turns toward the prism apex.

To carry the experiment further, the following test may be employed on a patient. Covering one eye, direct his attention to a fixed object, placing the ten degree prism before the eye, but far enough away to see the patient’s eye behind it. As the prism is brought in to the line of vision, it will be seen that the eye turns towards the apex of the prism. When the prism is removed, the eye returns to its normal position.

Similar experiments enable the refractionist to make the most practical use of treating phorias and ductions, as well as to comprehend all other technical work.

Fig. 13—An important part of the equipment for muscular work.

The Phorometer

As previously stated, it is practically impossible to accurately diagnose a case of muscular imbalance with trial-case prisms. For this reason the phorometer forms an important part of the equipment for muscle testing in the Ski-optometer, having proven both rapid and accurate. It consists of two five-degree prisms with bases opposite, each reflecting an object toward the apex or thin edge. The patient whose attention is directed to the usual muscle-testing spot of light, will see two spots.

Aside from the instrument itself, and in further explanation of the phorometer’s principle and construction, when two five-degree prisms are placed together so that their bases are directly opposite, they naturally neutralize; when their bases are together, their strength is doubled. Thus while the prisms of the phorometer are rotating, they give prism values from plano to ten degrees, the same being indicated by the pointer on the phorometer’s scale of measurements.

As a guide in dark-room testing, it should be noted that the handle of the phorometer in a vertical position is an indication that the vertical muscles are being tested; if horizontal, the horizontal muscles are undergoing the test.

The Maddox Rod

The Maddox rod ([Fig. 14]) consists of a number of red or white rods, which cause a corresponding colored streak to be seen by the patient. This rod is placed most conveniently on the instrument, being provided with independent stops for accurately setting the rods at 90 or 180 degree positions. The Maddox rod has proven of valuable assistance in detecting muscular defects, particularly when used in conjunction with the phorometer. Thus employed, it enables the patient to determine when the streak seen with one eye crosses through the muscle-testing spot-light observable by the other eye, as hereafter described.

Fig. 14—The Maddox Rod,
a valuable aid in making muscular tests.

Procedure for Making the Muscle Test

The Ski-optometer should be equipped with two Maddox rods, one red and one white. Their combined use is of the utmost importance since they assist in accurately determining cyclophoria and its degree of tortion as designated on the degree scale, and fully described in a later chapter.

When the Maddox rods are placed in a vertical position, it is an indication that the vertical muscles are being tested; when placed horizontally, the horizontal muscles are being tested. It should be particularly noted that the streaks of light observable through the Maddox rods always appear at right angles to the position in which they lie.

The Ski-optometer should be placed in a comfortable position before the patient’s face with the brow-rest and pupillary distance adjusted to their respective requirements. The instrument should be levelled so that the bubble of the spirit level lies evenly between its two lines, thus insuring horizontal balance. The muscle test light should be employed at an approximate distance of twenty feet on a plane with the patient’s head. Best results in muscle testing are secured through the use of the Woolf ophthalmic bracket, with iris diaphragm chimney and a specially adapted concentrated filament electric lamp ([Fig. 9]). This gives a brilliant illumination which is particularly essential. The test for error of refraction should be made in the usual manner, using the spherical and cylindrical lenses contained in the Ski-optometer, thus obviating the transference of trial-case lenses and the use of a cumbersome trial-frame. The time-saving thus effected enables the refractionist to include a muscle test in every examination and without tiring the patient—a consideration of the utmost importance.

Binocular and Monocular Test

The test for muscular imbalance may be divided in two parts. First, binocular test, or combined muscle test of the two eyes; second, monocular test, or muscle test of each eye separately. The latter does not signify the shutting out of vision or closing off of either eye, since muscular imbalance can only be determined when both eyes are open. These two tests are fully explained in the following chapter.

Chapter VII
THE BINOCULAR MUSCLE TEST

Made with the Maddox Rod
and Phorometer

Directing the patient’s attention to the usual muscle testing spot of light, the red Maddox rod should be placed in operative position before the eye, with the single white line or indicator on red zero ([Fig. 15]). The rods now lie in a vertical position.

Fig. 15—The Maddox rods placed vertically denote test for right or left hyperphoria, causing a horizontal streak to be seen by patient.

The pointer of the phorometer should likewise be set on the neutral line of the red scale, causing the handle to point upward ([Fig. 16]). A distance point of light and a red streak laying in a horizontal position should now be seen by the patient.

Fig. 16—The phorometer handle placed vertically, denotes vertical muscles are undergoing test for right or left hyperphoria—as indicated by “R. H.” or “L. H.”

Instead of memorizing a vast number of rules essential where trial case prisms are employed for testing ocular muscles, the pointer of the phorometer indicates not only the degree on the red scale, but the presence of right hyperphoria (R. H.) or left hyperphoria (L. H.).

Fig. 17—The horizontal streak caused by Maddox rod bisecting muscle testing spot-light for vertical imbalance, as patient should see it.

Assuming that the patient finds that the streak cuts through the point of light, the refractionist instantly notes the absence of hyperphoria. Should the point of light and the red streak not bisect, prism power must be added by rotating the phorometer’s handle to a position that will cause the streak to cut through the light ([Fig. 17]). While testing for hyperphoria, the red scale should alone be employed, the white scale being totally ignored.

Fig. 18—The Maddox rods placed horizontally test esophoria or exophoria, causing a vertical streak to be seen by the patient.

Esophoria and Exophoria

The next step is to set the white lines of the red Maddox rod either at white zero, or 180° line, with the rods in a horizontal position ([Fig. 18]) and the phorometer on the white neutral line, with handle horizontal ([Fig. 19]), thus making the test for esophoria or exophoria, technically known as lateral deviations.

The red streak will now be seen in a vertical position. Should it bisect the spot of light, it would show that no lateral imbalance exists. Should it not bisect, the existence of either esophoria or exophoria is proven, necessitating the turning of the phorometer handle. Should the refractionist rotate the handle in a direction opposing that of the existing imbalance, the light will be taken further away from the streak, indicating that the rotation of the prisms should be reversed.

Fig. 19—The phorometer handle placed horizontally denotes horizontal muscles are undergoing test for esophoria or exophoria indicated by “Es.” or “Ex.”

At the point of bisection ([Fig. 20]), the phorometer will indicate on the white scale whether the case is esophoria or exophoria and to what amount. In testing esophoria (ES) or exophoria (EX), the white scale is alone employed, no attention being given to the red scale.

Fig. 20—The vertical streak bisecting muscle testing spot-light for horizontal imbalance, as patient should see it.

Making Muscle Test Before and
After Optical Correction

It is considered best to make the binocular test before regular refraction is made, making note of the findings; and again repeating the test after the full optical correction has been placed before the patient’s eye. This enables the refractionist to definitely determine whether the correction has benefited or aggravated the muscles. Furthermore, by making the muscle test before and after the optical correction, a starting point in an examination is frequently attained. For example, where the phorometer indicates esophoria it is usually associated with hyperopia, whereas exophoria is usually associated with myopia, thus serving as a clue for the optical correction.

Assuming for example that the binocular muscle test shows six degrees of esophoria without the optical correction, and with it but four degrees, it is readily seen that the imbalance has been benefited by the optical correction. Under such conditions it is safe to believe that the optical correction will continue to benefit as the patient advances in years, tending to overcome muscular defect.

When to Consider Correction
of Muscular Imbalance

In correcting an imbalance, it is also a good plan to adhere to the following rule: In case of hyperphoria, either right or left, consider for further correction only those cases that show one degree or more. In exophoria, those showing three degrees or more. In esophoria, correct those showing five degrees or more, except in children, where correction should be made in cases showing an excess of 3° of esophoria. These rules are naturally subject to variation according to the patient’s refraction and age, but they are generally accepted as safe.

Four Methods for Correction
of Muscular Imbalance

There are four distinct methods for correcting muscular imbalance, each of which should be carried out in the following routine:

1. Optical correction made with spheres or cylinders, or a combination of both.

2. Muscular exercising or “ocular gymnastics.” This is accomplished on the same principle as the employment of other forms of exercises, or calisthenics.

3. The use of Prisms: When the second method fails, prisms are supplied, with base of prism before the weak muscle, for rest only.

4. Operation: If the above three methods, as outlined in the following chapters, have been carefully investigated, nothing remains but a tetonomy or advancement, or other operative means for relief and satisfaction to the patient.

The Rotary Prism

The rotary prism of the Ski-optometer, ([Fig. 21]) consists of a prism unit, having a total equivalent of thirty degrees. It is composed of two fifteen-degree prisms, back to back, so that the turn of its pinion or handle causes each of its lenses to revolve, one on the other. When its bases are opposite, they neutralize; when directly together, they give a total value of thirty degrees. While revolving from zero to maximum strength, they give prism values which are indicated on the scale of measurements, the red line denoting the total prism equivalent.

Fig. 21—Turning rotary prism’s pinioned handle gives prism value from zero to 30° as indicated by prism’s red line indicator.

It is obviously essential to know where the base of the rotary prism is located. Therefore if prism in or out is desired, the zero graduations should be placed vertically and the red line or indicator set at the upper zero ([Fig. 21]).

A rotation inward to 10 would give a prism equivalent of ten degrees, base in. A rotation from zero to 10 outward would give a prism equivalent of ten degrees, base out, etc. With zero graduations horizontal and the red line or indicator set therewith, a rotation upward to ten on the scale would give a prism equivalent of ten degrees, base up. A rotation from zero downward to 10 would give a prism equivalent of ten degrees, base down.

An understanding of the foregoing will show that a rotation of the red line, or indicator, will give prism value from zero to 30, with base up, down, in or out.

Use of the Rotary Prism in
Binocular Muscle Tests

Should a case be one of esophoria, exceeding the ten degree range of the phorometer, the rotary prism should be brought into operative position with cypher (0) graduations vertical ([Fig. 21]), while the red line or indicator should be set at 10 on the outer or temporal scale. The phorometer’s indicator should again be set on the center or neutral line on the white scale. The rotary prism will then add ten degrees to the esophoria reading indicated on the phorometer.

Should the case be one of exophoria, exceeding ten degrees, the indicator should be set at ten degrees upon the inner or nasal scale and the indicator of the phorometer should then be set at the white center or neutral line, as in the previous test. Should prism power ever be required to supplement the phorometer in hyperphoria, the rotary prism should be employed with zero graduations horizontal, and the red line or indicator set at ten degrees on upper or lower scale, as required.

Chapter VIII
THE MONOCULAR DUCTION MUSCLE TEST

Made with Both Rotary Prisms

While the previously described binocular muscle test made with the phorometer and Maddox rod, only determines the existence and amount of esophoria, exophoria, and hyperphoria, neither the faulty nor the deviating muscle is located, hence a monocular muscle test is essential in order to determine whether the muscles of the right or left eye are faulty. Furthermore, an imbalance may possibly be due to either a faulty muscular poise, or lack of nerve force in one or both eyes. A “duction test” should accordingly be made of each muscle of each eye separately, followed by a comparison of the muscular pull of both eyes collectively.

These tests are commonly termed adduction, abduction, superduction and subduction, and are defined in the order named. They include tests of the vertical and horizontal muscles of each eye, made individually by means of the rotary prisms, each being placed before the eye undergoing the test.

Locating the Faulty Muscle

The phorometer and the Maddox rod should be removed from operative position, discontinuing the use of the muscle-testing spot-light, employed in the previously described binocular test. The optical correction, if one is required, should be left in place, while the patient’s attention should be directed, with both eyes open, to the largest letter on the distant test chart; or if preferable, the Greek cross in the Woolf ophthalmic, chimney may be used. Either one, however, should be located on a plane with the patient’s head. As a guide for the operator, it might be well to remember that when the handle of the rotary prism is in a horizontal position, the lateral or horizontal muscles are being tested. On the other hand, when the handle is in a vertical position, the vertical muscles are undergoing the test.

Adduction

Adduction, or relative convergence, is the power of the internal muscles to turn the eyes inward; prism power base out and apex in, is employed.

Fig. 22—To test adduction, base out is required. Rotary prism’s line or indicator should be rotated from zero outwardly.

To test abduction, base in is required. Indicator should be rotated inwardly from zero.

To test adduction of the patient’s right eye, the rotary prism should be placed in position before the right eye, the red line or prism indicator being registered at zero upon the prism upper scale. The two cyphers (0) should be placed in a vertical position with the handle pointed horizontally ([Fig. 21]). The rotary prism should then be rotated so that its red line or indicator is rotated outward from zero until the large letter—preferably the largest letter, which is usually “E”—on the distance test-type or the Greek cross previously referred to, first appears to double in the horizontal plane. The reading on the scale of measurements should accordingly be noted. This test should be repeated several times, constantly striving for the highest prism power that the patient will accept without producing diplopia. The prism equivalent thus obtained will indicate the right adduction and should be so recorded, as designated in [Fig. 24]. The amount of adduction ranges from 6 to 28, prism diopters, the normal average being 24.

Abduction

Abduction is the relative power of the external muscles to turn the eyes outward. Prism power base in and apex out is employed. To determine abduction, or the amount of divergence of the external rectus muscle of the right eye, prism power with base in or toward the nasal side should be employed. The rotary prism will therefore remain in the same relative position as in making the adduction test ([Fig. 22]), with the two cyphers (0) or zero graduations vertical, but the indicator or red line should be rotated inward from zero, or towards the patient’s nose.

With the patient’s attention again directed to the large letter “E,” or the Greek cross, this inward rotation should be continued until diplopia or double vision occurs. Like the former, this test should be repeated several times, the refractionist continuing to strive for the highest prism power which the eye will accept. This will indicate abduction of the right eye and should be so recorded as designated in [Fig. 24]. The amount of abduction ranges from 3 to 10 prism diopters. The normal average is 8.

The ratio of adduction to abduction is normally rated at about three to one. In other words, it is conceded that the power of the eye to converge is normally three times as great as its power to diverge, the usual measurements being eight to twenty-four respectively. While applicable in most instances, this may vary in different cases.

Superduction

Superduction, sometimes termed sursumduction, is the relative power of the superior recti to turn the eyes upward. Prism power base down and apex up is employed. To test superduction, the rotary prism should be placed in position with the two cyphers lying horizontally, with the handle pointed vertically ([Fig. 23]). The patient’s attention should again be directed to the large letter “E”, and the indicator or red line should be rotated downward from zero. The highest prism power that the patient will accept before the object appears to double in the vertical plane will indicate the degree of right superduction. This should be recorded accordingly. Conditions of this kind do not usually exceed two or three degrees. The test, however, should be repeated several times before the final result is recorded, as indicated in [Fig. 24]. The amount of superduction ranges from 1 to 4 prism diopters. The normal average is 2.

Fig. 23—To test superduction, base down is required. Rotary prism’s line or indicator should be rotated downward from zero.

To test subduction, base up is required. Indicator should be rotated upward from zero.

Subduction

Subduction, sometimes termed infraduction or deorsumduction, is the relative power of the inferior recti to turn the eyes downward. Prism power base up and apex down is employed. To test subduction, the rotary prism should be operated with zero graduations placed horizontally, as in the superduction test ([Fig. 23]), but the indicator should be slowly rotated in the reverse direction, or upward from zero. With the patient’s attention again directed to the large letter “E,” or the Greek cross, the strongest degree prism thus secured without diplopia will indicate the right subduction. The amount of subduction ranges from 1 to 4 prism diopters. The normal average is 2.

Any difference between superduction and subduction, usually denoting the existence of hyperphoria, should be given careful consideration.

Procedure for Monocular Muscle Testing

As previously explained, after a duction test of each of the four muscles of the right eye, the rotary prism before that eye should be placed out of position and the procedure for adduction, abduction, superduction and subduction repeated by means of the rotary prism before the left eye. In case of an existing imbalance, after testing the muscle of both right and left eyes, the refractionist can quickly determine which muscle or muscles may be lacking in strength ([Fig. 24]). In practically every instance muscle exercises or correcting prisms may then be prescribed with definite knowledge of requirements, as further described in the following paragraphs.

A binocular muscle test made with the phorometer, Maddox rod and distant muscle-testing point of light might quickly indicate six degrees of exophoria, both before and after the optical correction is made. While this would doubtless be the correct amount of the manifest imbalance, it would be a difficult matter to ascertain which muscles caused the disturbance. To determine this important question, the monocular or duction test should be invariably employed.

Diagnosing a Specific Muscle Case

Assuming, for example, a specific case where six degrees of exophoria was determined in the binocular test that the muscle findings in the duction test show right adduction of twenty-four degrees, with an accompanying abduction of eight degrees; likewise a superduction and subduction of two degrees for each eye. With the aid of a chart or diagram—which should be made in every case—a comparison of these figures would indicate an exophoria of approximately six degrees, with a corresponding weak left internus ([Fig. 24]). This not only shows the muscle pull of each eye individually, but a comparison of the two eyes as indicated by the dotted lines. Thus the relationship of the two eyes, and their corresponding muscles is quickly indicated.

Fig. 24—Duction chart should be made in every case. Above readily shows existence of muscular imbalance and proves subduction and superduction for both eyes are equal; otherwise hyperphoria would be disclosed. Also note abduction for both right and left eye are equal, otherwise esophoria would be disclosed. Also note adduction for right eye is 24° while left is but 18°, proving a case of 6° of exophoria with a left weak internus.

A glance at the above diagram discloses the following three important facts, all of which should be known to the refractionist before a single thought can be devoted to the correcting of the case:

1. 6° exophoria is the amount of the insufficiency.

2. 18° adduction (which should be 24°).

3. Left weak internus.

As previously stated, the power to converge is normally rated 3 to 1, or 8 to 24, as shown above, while the power of the eye to look upward, is equal to the power to look downward. The diagram accordingly proves that the muscles of the right eye are in perfect balance, having equal muscular energy.

A comparison of the left eye shows adduction of 18 degrees with an abduction of 8 degrees, proving a lateral insufficiency because the ratio is less than 3 to 1; and the muscles of the left eye are at fault. The power of 2 degrees superduction and 2 degrees subduction, proves that no weakness exists in the vertical muscles.

After making the duction test for each eye individually, a comparison of both eyes in relationship to each other may be more readily determined by following the dotted lines ([Fig. 24]).

As previously stated, it is the inability of the two eyes to work together that causes the imbalance, so that if both eyes were normal, the adduction, abduction, superduction and subduction of the two eyes would agree.

The duction chart ([Fig. 24].) also shows that the corresponding muscles of each eye agree—with the exception of the adduction of the right eye and the left eye. This proves that the left internus is weak, measuring only 18 degrees instead of 24 degrees; it further proves the 6 degrees of exophoria in the monocular test, as was quickly and more readily determined in the binocular test.

Likewise, in cases of esophoria, hyperphoria, or cataphoria, the making of definite muscle measurements independently through the prescribed method would show through the merest glance at a similar diagram which muscle or muscles were relatively out of balance. Heterphoria of almost any type, or tendencies other than normal, may be fully investigated by making a thorough and separate test of each muscle.

Where an imbalance exists, a rapid test may be employed to distinguish a pseudo or false condition from a true condition. This is accomplished by first placing the two Maddox rods (both the red and white) so that the rods lie in a vertical position. If the two lines fuse, we have determined the existence of a false condition caused by a possible error of refraction or nerve strain. If the lines separate, we have determined a true muscular condition, and then only should the second method of muscular treatment follow.