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• [PREFACE]
• [CONTENTS]
• [BIBLIOGRAPHY]
• [INDEX]

Monographs On Experimental Biology

EDITED BY

JACQUES LOEB, Rockefeller Institute
T. H. MORGAN, Columbia University
W. J. V. OSTERHOUT, Harvard University

THE NATURE OF ANIMAL LIGHT

BY

E. NEWTON HARVEY, Ph.D.

MONOGRAPHS ON EXPERIMENTAL BIOLOGY

PUBLISHED

FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT
By JACQUES LOEB, Rockefeller Institute

THE ELEMENTARY NERVOUS SYSTEM
By G. H. PARKER, Harvard University

THE PHYSICAL BASIS OF HEREDITY
By T. H. MORGAN, Columbia University

INBREEDING AND OUTBREEDING: THEIR GENETIC AND SOCIOLOGICAL SIGNIFICANCE
By E. M. EAST and D. F. JONES, Bussey Institution, Harvard University

THE NATURE OF ANIMAL LIGHT
By E. N. HARVEY, Princeton University

IN PREPARATION

PURE LINE INHERITANCE
By H. S. JENNINGS, Johns Hopkins University

THE EXPERIMENTAL MODIFICATION OF THE PROCESS OF INHERITANCE
By R. PEARL, Johns Hopkins University

LOCALIZATION OF MORPHOGENETIC SUBSTANCES IN THE EGG
By E. G. CONKLIN, Princeton University

TISSUE CULTURE
By R. G. HARRISON, Yale University

PERMEABILITY AND ELECTRICAL CONDUCTIVITY OF LIVING TISSUE
By W. J. V. OSTERHOUT, Harvard University

THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN ORGANISM AND ENVIRONMENT
By L. J. HENDERSON, Harvard University

CHEMICAL BASIS OF GROWTH
By T. B. ROBERTSON, University of Toronto

COÖRDINATION IN LOCOMOTION
By A. R. MOORE, Rutgers College

OTHERS WILL FOLLOW

Monographs on Experimental Biology

THE NATURE OF ANIMAL LIGHT

BY

E. NEWTON HARVEY, Ph.D.

PROFESSOR OF PHYSIOLOGY, PRINCETON UNIVERSITY

PHILADELPHIA AND LONDON J. B. LIPPINCOTT COMPANY

COPYRIGHT, 1920. BY J. B. LIPPINCOTT COMPANY

Electrotyped and Printed by J. B. Lippincott Company.
The Washington Square Press, Philadelphia, U. S. A.

EDITORS' ANNOUNCEMENT

The rapid increase of specialization makes it impossible for one author to cover satisfactorily the whole field of modern Biology. This situation, which exists in all the sciences, has induced English authors to issue series of monographs in Biochemistry, Physiology, and Physics. A number of American biologists have decided to provide the same opportunity for the study of Experimental Biology.

Biology, which not long ago was purely descriptive and speculative, has begun to adopt the methods of the exact sciences, recognizing that for permanent progress not only experiments are required but quantitative experiments. It will be the purpose of this series of monographs to emphasize and further as much as possible this development of Biology.

Experimental Biology and General Physiology are one and the same science, in method as well as content, since both aim at explaining life from the physico-chemical constitution of living matter. The series of monographs on Experimental Biology will therefore include the field of traditional General Physiology.

Jacques Loeb,
T. H. Morgan,
W. J. V. Osterhout.

PREFACE

Bioluminescence, the production of light by animals and plants, has always excited the admiration of the layman and the wonder of the scientist. It is not surprising that an enormous literature dealing with the subject has grown up. A large part of this literature, however, is made up merely of reports that a certain animal is luminous, or records of especially brilliant phosphorescence of the sea. Among those who have inquired somewhat more carefully into the nature and causes of light production may be mentioned the names of Beijerinck, R. Boyle, Dahlgren, Dubois, Ehrenberg, Krukenberg, Mangold, McDermott, Molisch, Panceri, Pflüger, Phipson, Quatrefages, Spallanzani, and Trojan. Several of these men have written comprehensive monographs on the subject.

It is not the purpose of this book to deal with every phase of bioluminescence. Volumes could be written on the evolutionary side of the problem and the structure and uses of luminous organs. These questions can only be touched upon. Neither is it my purpose to discuss the ultimate cause of the light, whether due to vibration of electrons or to other causes. That problem must be left to the physicist, although it is highly probable that a study of animal light will give important information regarding the nature of light in general, and no theory of light can be adequate which fails to take into account the extraordinary powers of luminous animals.

We shall be concerned largely with the physical characteristics of animal light and the chemical processes

underlying its production. Great advances have been made since the first early guesses that the light was due to phosphorus and was a kind of oxidation. Although the problem cannot be considered as solved, it has been placed on a sound physico-chemical basis. Some material is oxidized. Exactly what this material is and why light accompanies its oxidation are the two more fundamental problems in the field of Bioluminescence. How far and with what success we have progressed toward a solution of these problems may be seen from a perusal of the following pages.

It gives me pleasure to acknowledge the kindness of Dr. W. E. Forsythe of the Nela Institute, Cleveland, Ohio, in reading and criticizing the manuscript of [Chapter III], and of Professor Lyman of Harvard University for a similar review of [Chapter II]. I am also deeply indebted to my wife for reading the proof and to Dr. Jacques Loeb and Prof. W. J. V. Osterhout for many suggestions throughout the book. My thanks are also due to Prof. C. Ishikawa of the Agricultural College, Imperial University of Tokio, Japan, for his generous assistance in providing Cypridina material. Finally I wish to acknowledge the support of the Carnegie Institution of Washington, through its director of Marine Biology, Dr. Alfred G. Mayor. Without this support much of the work described in this book could not have been accomplished.

E. N. H.
Princeton, N. J.,
October, 1919.

CONTENTS

THE NATURE OF ANIMAL LIGHT

CHAPTER I
LIGHT-PRODUCING ORGANISMS

The fact that animals can produce light must have been recognized from the earliest times in countries where fireflies and glowworms abound, but it is only since the perfection of the microscope that the phosphorescence of the sea, the light of damp wood and of dead fish and flesh has been proved to be due to living organisms. Aristotle mentions the light of dead fish and flesh and both Aristotle and Pliny that of damp wood. Robert Boyle in 1667 made many experiments to show that the light from all three sources, as well as that of the glowworm, is dependent upon a plentiful supply of air and drew an interesting comparison between the light of shining wood and that of a glowing coal. Boyle had no means of finding out the true cause of the light and early views of its nature were indeed fantastic. Even as late as 1800 Hulme concludes from his experiments on phosphorescent fish that the light is a "constituent principle of marine fishes" and the "first that escapes after the death of the fish." It was only in 1830 that Michaelis suspected the light of dead fish to be the result of some living thing and in 1854 Heller gave the name Sarcina noctiluca to the suspected organism. In 1875 Pflüger showed that nutrient media could be inoculated with small amounts of luminous fish and that these would increase in size, like bacterial

colonies, and we now know that the light of all dead fish and flesh is due to luminous bacteria.

In the early part of the nineteenth century it was surmised that the light of damp wood was connected with fungus growth because of a similarity in smell. In 1854 Heller recognized minute strands, which he called Rhizomorpha noctiluca, as the actual source of the light. We now know that all phosphorescent wood is due to the mycelium of various kinds of fungi and that sometimes the fruiting body of the fungus also produces light.

The phosphorescence or "burning of the sea," which is described by so many of the older explorers, is also due entirely to living organisms, both microscopic and macroscopic. The latter are mostly jelly-fish (medusæ) or comb jellies (Ctenophores) and give rise to the larger, more brilliant flashes of light often seen in the wake or about the sides of a steamer at night. The former are various species of dinoflagellates or cystoflagellates such as Noctiluca (just visible to the naked eye) which collect at the surface of the sea and often increase in such numbers that the water is colored by day (usually pink or red) and shines like a sheet of fire when disturbed at night. Although Noctiluca was recognized as a luminous animal in 1753 by Baker, the light of the sea was a mysterious phenomenon to the older observers. MacCartney, speaking before the Royal Society in 1810, outlines the various older theories as follows: "Many writers have ascribed the light of the sea to other causes than luminous animals. Martin supposed it to be occasioned by putrefaction; Silberschlag believed it to be phosphoric; Prof. J. Mayer conjectured that the surface of the sea imbibed light, which it afterwards discharged. Bajon and Gentil

thought the light of the sea was electric, because it was excited by friction.... I shall not trespass on the time of the Society to refute the above speculations; their authors have left them unsupported by either arguments or experiments, and they are inconsistent with all ascertained facts upon the subject. The remarkable property of emitting light during life is only met amongst animals of the four last classes of modern naturalists, viz., mollusca, insects, worms, and zoöphytes." MacCartney recognized the true cause of the light, although he had little idea of the vast number of marine forms which are luminous and omits entirely any reference to the fishes, many of which produce a light of their own when living, apart from any bacterial infection.

A survey of the animal kingdom discloses at least 36 orders containing one or more forms known to produce light and several more orders containing species whose luminosity is doubtful. In the plant kingdom there are two groups containing luminous forms. The distribution of luminous organisms is brought out in the [accompanying classification of plants and animals]. Those orders are printed in italics which contain species whose self-luminosity is fairly well established. It will be noted that further subdivisions into orders is not given in classes of animals which lack luminous forms.

TABLE 1
DISTRIBUTION OF LUMINOUS ORGANISMS IN PLANT AND ANIMAL KINGDOMS

Plant Kingdom

• I. Thallophyta
• Algæ
• Cyanophyceæ (Blue-green Algæ)
• Chlorophyceæ (Green Algæ)
• Phæophyceæ (Brown Algæ)
• Rhodophyceæ (Red Algæ)
• Lichenes (Lichens, symbiotic growth of algæ and fungi)
• Fungi
• Myxomycetes (Slime moulds)
• Schizomycetes (Bacteria)
• Bacterium, Photobacterium, Bacillus, Pseudomonas, Micrococcus, Microspira, Vibrio.
• Phycomycetes (moulds)
• Ascomycetes (Sac fungi, yeasts, some moulds)
• Basidiomycetes (Smuts, rusts, mushrooms)
• Ustilaginæ (Smuts)
• Uridineæ
• Auriculariæ (Judas ears)
• Tremellineæ (Jelly fungi)
• Hymenomycetes (Mushrooms)
• Agaricus, Armillaria, Pleurotus, Panus, Mycena, Omphalia, Locellina, Marasinium, Clitocybe, Corticium.
• Gasteromycetes (Stinkhorns and puff-balls)

• II. Bryophyta
• Hepaticæ (Liverworts)
• Musci (Mosses)

• III. Pteridophyta
• Equisetineæ (Horsetails)
• Salviniæ (Salvinia, Marsilia, etc.)
• Lycopodineæ (Club Mosses)
• Filicineæ (Ferns)

• IV. Spermatophyta
• Gymnospermæ (Cycads, Ginkgo, Conifers)
• Angiospermæ (Mono- and Dicotyledonous flowering plants).

Animal Kingdom

• I. Protozoa. (One-celled animals)
• Sarcodina
• Rhizopoda
• Heliozoa
• Radiolaria
• Thallassicola, Myxosphæra, Collosphæra, Collozoum, Sphærozoum.
• Mastigophora
• Flagellata
• Choanoflagellata
• Dinoflagellata
• Ceratium, Peridinium, Prorocentrum, Pyrodinium, Gonyaulax, Blepharocysta, Amphidinium, Diplopsalis, Cochlodinium, Sphærodinium, Gymnodinium.
• Cystoflagellata
• Noctiluca, Pyrocystis, Leptodiscus, Craspedotella.
• Sporozoa
• Infusoria

• II. Porifera (Sponges)
• Calcarea
• Hexactinellida
• Desmospongiæ

• III. Cœlenterata
• Hydrozoa (Hydroids and Jelly-fish)
• Leptomedusæ or Campanulariæ
• Medusa form—Eutima, Phyalidium (Oceania).
• Hydroid form—Aglaophenia, Campanularia, Sertularia, Plumularia, Cellularia, Valkeria, Obelia, Clytia.
• Trachomedusæ
• Geryonia, Lyriope, Aglaura
• Narcomedusæ
• Cunina
• Anthomedusæ or Tubulariæ
• Medusa form—Thaumantias, Tiara, Turris, Sarsia.
• Hydroid form—?
• Hydrocorallinæ
• Siphonophora
• Abyla, Praya, Diphyes, Eudoxia, Hippopodius.
• Scyphozoa (Jelly-fish)
• Stauromedusæ
• Peromedusæ
• Cubomedusæ
• Carybdia
• Discomedusæ
• Pelagia, Aurelia, Chrysaora, Rhizostoma, Cyanæa, Dianea, Mesonema.
• Actinozoa (Corals, Sea-fans, Sea-pens, Sea-anemones)
• Actinaria
• Madreporareia
• Antipatharia
• Alcyonaria
• Alcyonium, Gorgonia, Isis, Mopsea
• Pennatulacea
• Pennatula, Pteroides, Veretillum, Cavernularia.
• Funicularia, Renilla, Pavonaria, Stylobelemon, Umbellularia, Virgularia?
• Ctenophora (Comb-jellies)
• Cydippida
• Pleurobranchia.
• Lobata
• Mnemiopsis, Bolinopsis, Leucothea (Eucharis).
• Cestida
• Cestus.
• Beroida
• Beroë.

• IV. Platyhelminthes
• Turbellaria (Flat-worms)
• Trematodes (Parasitic flat-worms)
• Cestodes (Tape-worms)
• Nemertinea (Nemertines)

• V. Nemathelminthes
• Nematoda (Round worms)
• Gordiacea (Hair worms)
• Acanthocephala (Acanthocephalids)
• Chætognatha (Sagitta)

• VI. Trochelminthes
• Rotifera (Wheel animalcules)
• Gastrotricha (Chætonotus)
• Kinorhyncha (Echinoderes)

• VII. Molluscoidea
• Bryozoa (Corallines)
• Entoprocta
• Ectoprocta
• Membranipora, Scrupocellaria, Retepora? Flustra?
• Brachiopoda (Lamp shells)
• Phoronidea (Phoronis)

• VIII. Annulata
• Archiannelida (Primitive worms, including Dinophilus)
• Chætopoda (True worms)
• Polychæta
• Chætopterus, Phyllochaetopterus, Telepsaris, Polynoë, Acholoë, Tomopteris, Odontosyllis, Lepidonotus, Pionosyllis, Phyllodoce, Heterocirrus, Polyopthalamus?
• Oligochæta
• Lumbricus, Photodrilus, Allolobophora (Eisemia), Microscolex, Nonlea, Enchytræus, Octochætus.
• Gephyrea (Sipunculus)
• Hirudinea (Leeches)
• Myzostomida (Myzostomus)

• IX. Echinodermata
• Asteroidea (Star-fish)
• Ophiuroidea (Brittle-stars)
• Ophiurida
• Ophiopsila, Amphiura, Ophiacantha, Ophiothrix, Ophionereis.
• Euryalida
• Echinoidea (Sea urchins)
• Holothuroidea (Sea Cucumbers)
• Crinoidea (Feather-stars)

• X. Arthropoda
• Crustacea (Crabs, lobsters, shrimps, etc.)
• Phyllapoda
• Ostracoda
• Halocypris, Cypridina, Pyrocypris, Conchœcia, Cyclopina.
• Copepoda
• Metridia, Leuckartia, Pleuromma, Oncæa, Heterochæta.
• Cirripedia
• Phyllocardia
• Schizopoda
• Nyctiphanes, Nematoscelis, Gnathophausia, Euphausia, Stylochiron,Boreophausia, Mysis?
• Decapoda
• Sergestes, Aristeus, Heterocarpus, Hoplophorus, Acanthephyra, Pentacheles, Colossendeis
• Stomatopoda
• Cumacea
• Amphipoda
• Isopoda
• Onychophora (Peripatus)
• Myriapoda (Centipedes and Millepedes)
• Symphyla
• Chilopoda
• Geophilus, Scolioplanes, Orya.
• Diplopoda
• Pauropoda
• Insecta (Insects)
• Aptera (Spring-tails)
• Lipura, Amphorura, Neanura
• Orthoptera
• Neuroptera
• Teleganoides and Cænis of the Mayflies? Termites?
• Hemiptera
• Diptera (Flies)
• Bolitophila and Ceroplatus larvæ, Thyreophora?
• Coleoptera (Beetles)
• Pyrophorus, Photophorus, Luciola, Lampyris, Phengodes, Photuris, Photinus, etc.
• Lepidoptera
• Hymenoptera
• Arachnida (Spiders)

• XI. Mollusca
• Amphineura (Chiton)
• Pelecypoda (Bivalves)
• Protobranchia
• Filibranchia
• Pseudo-Lamellibranchia
• Eu-lamellibranchia
• Pholas
• Septibranchiata
• Gasteropoda (Snails, periwinkles, slugs, etc.)
• Prosobranchiata
• Ophisthobranchiata
• Phyllirrhoë, Plocamopherus.
• Pulmonata
• Scaphopoda (Dentalium)
• Cephalopoda (Squids and Octopus)
• Tetrabranchiata
• Dibranchiata decapoda
• Onychoteuthis, Chaunoteuthis, Lycoteuthis, Nematolampas, Lampadioteuthis, Enoploteuthis, Abralia, Abraliopsis, Watasenia, Ancistrocheirus, Thelidioteuthis, Pterygioteuthis, Pyroteuthis, Octopodoteuthis?, Calliteuthis, Histioteuthis, Benthoteuthis, Hyaloteuthis, Eucleoteuthis, Chiroteuthis,
• Mastigoteuthis, Cranchia, Liocranchia, Pyrgopsis, Leachia, Liguriella, Phasmatopsis, Toxeuma, Megalocranchia, Leucocranchia, Crystalloteuthis, Phasmatoteuthis, Galiteuthis, Corynomma, Hensenioteuthis, Bathothauma, Rossia?, Heteroteuthis, Iridoteuthis, Sepiola, Rondeletia, Inioteuthis, Euprymna, Melanoteuthis?.

• XII. Chordata
• Adelochorda (Balanoglossus)
• Balanoglossus, Ptychodera, Glossobalanus
• Urochorda (Ascidians)
• Larvacea
• Appendicularia?
• Thaliacea
• Salpa, Doliolum?
• Ascidiacea
• Pyrosoma, Phallusia
• Acrania (Amphioxus)
• Cyclostomata (Cylostomes)
• Pisces (Fishes)
• Elasmobranchii
• Centroscyllium, Spinax, Paracentroscyllium, Isistius, Læmargus, Euproctomicrus, Benthobatis?
• Holocephalii
• Dipnoi
• Teleostomi
• Stomias, Chauliodus, Melanostomius, Pachystomias, Bathophilus, Dactylostomius, Malacosteus, Astronesthes, Ophozstomias, Idiacanthus, Bathylychnus, Macrostomius, Gonostoma, Cyclothone, Photichthys, Vinciguerria, Ichthyococcus, Lychnopoles, Diplophos, Triplophos, Valenciennellus, Maurolicus, Argyropelecus, Sternoptyx, Polyipnus, Ipnops? Neoscopelus, Myctophum, Halosausus, Xenodermichthys? Macrurus? Photoblepharon, Anomalops, Porichthys, Leuciocornus, Mixonus? Bassozetus? Oneirodes, Ceratias, Gigantactis, Chaunax, Malthopsis, Halicmetus, Monocentris, Lamprogrammus.
• Amphibia (Frogs, Toads, Salamanders)
• Reptilia (Snakes, Lizards, Turtles)
• Aves (Birds)
• Mammalia (Mammals)

The only groups of the plant kingdom which are known to produce light are some of the bacteria and some of the fungi and the dinoflagellates (Peridineæ) if one is to include them among the plants. Many different species of phosphorescent bacteria have been described, differing in cultural characteristics and structural peculiarities and grouped in the genera, Bacterium, Photobacterium, Bacillus, Microspira, Pseudomonas, Micrococcus, and Vibrio. Specific names indicating their light-producing power such as phosphorescens, phosphoreum, luminosum, lucifera, etc., have been applied.

All the fungi which are definitely known to produce light belong to the Basidiomycetes, the largest and most highly developed of the true fungi. Either the mycelium alone or the fruiting body alone, or both, may be luminescent.

Among animals the best known forms are the dinoflagellates; Noctiluca; hydroids; jelly-fish; ctenophores; sea pens; Chætopterus and other marine worms; earthworms; brittle stars; various crustaceans; myriapods; fireflies and glowworms, the larvæ of fireflies; Pholas dactylus and Phyllirrhoë bucephala, both molluscs; squid; Pyrosoma, a colonial ascidian; and fishes.

Luminous animals are all either marine or terrestrial forms. No examples of fresh water luminous organisms are known. Of marine forms, the great majority are deep sea animals, and it is among these that the development of true luminous organs of a complicated nature is most pronounced. Many of the luminous marine animals are to be found in the plankton, while the littoral luminous forms are in the minority. Some members of all the above groups are found at one or another of our marine labora

tories with the possible exception of Pholas, Phyllirrhoë and squid. Although earthworms and myriapods which produce light are found in the United States, they are rather rare and seldom observed forms.

Not only adult forms but the embryos and even the eggs of some animals are luminous. The egg of Lampyris emits light within the ovary and freshly laid eggs are quite luminous. The light does not come from luminous material of the luminous organ adhering to the egg when it is laid but from within the egg itself. Pyrophorus eggs are also luminous. The segmentation stages of Ctenophores are luminous on stimulation, as noted by Allman (1862), Agassiz (1874) and Peters (1905), but the eggs themselves do not luminesce. Schizopod larvæ (Trojan, 1907), Copepod nauplii (Giesbrecht, 1895), Chætopterus larvæ (Enders, 1909), and brittle star plutei (Mangold, 1907) also produce light.

Apparently there is no rhyme or reason in the distribution of luminescence throughout the plant or animal kingdom. It is as if the various groups had been written on a blackboard and a handful of sand cast over the names. Where each grain of sand strikes, a luminous species appears. The Cœlenterates have received most sand. Luminescence is more widespread in this phylum and more characteristic of the group as a whole than any other. Among the arthropods luminous forms crop up here and there in widely unrelated groups. In the mollusks, excluding the cephalopods, only two luminous species are known. Several phyla contain no luminous forms whatever. It is an extraordinary fact that one species in a genus may be luminous and another closely allied species contain no trace of luminosity. There seems to have been

no development of luminosity along direct evolutionary lines, although a more or less definite series of gradations with increasing structural complexity may be traced out among the forms with highly developed luminous organs.

While the accompanying list of luminous genera aims to be fairly complete, there are no doubt omissions and some inaccuracies in it. Anyone who has ever tried to determine what animal is responsible for the occasional flashes of light observed on agitating almost any sample of sea water will realize how difficult it is to discover the luminous form among a host of non-luminous ones, especially if the animal is microscopic in size. It is not surprising, then, to find many false reports of luminous animals in the literature of the subject and we cannot be too careful in accepting as luminous a reported case. The difficulty lies chiefly in the fact that all luminous organisms with the exception of bacteria, fungi, and a few fish, flash only on stimulation, and, while it is easy enough to see the flash, the animal is lost between the flashes. The only safe way to detect luminous organisms is to add a little ammonia to the sea water. This slowly kills the organisms and causes any luminous forms to glow with a steady, continuous light for some time, a condition accompanying the death of the animal. Not all observers, however, have followed this method. One must always be on guard against confusing the light from a supposed luminous form with the light from truly luminous organisms living upon it. The reported cases of luminosity among marine algæ are now known to be due to hydroids or unicellular organisms living on the alga.

We know also that many non-luminous forms may become infected with luminous bacteria, not only after

death, but also while living, so that their luminescence is purely secondary. Giard and Billet (1889-90) succeeded in inoculating many different kinds of amphipod crustacea (Talitrus, Orchestia, Ligia) and isopod crustacea (Porcellio, Philoscia) with luminous bacteria, in some cases passing the infection from one to the next through nine individuals. Curiously enough the bacterium did not produce light on artificial culture media but did when growing in the body of the crustacea, which were killed in about seven days by the infection. The species of Talitrus and Orchestia might easily have been taken for truly luminous animals if not carefully investigated.

Tarchanoff (1901) has injected luminous bacteria into the dorsal lymph sac of frogs with the result that the animals continued to glow for three to four days, especially about the tongue. I remember once while collecting luminous beetles in Cuba, I was astounded to find a frog which was luminous. Expecting this animal to be of great interest, I examined it further only to find that the frog had just finished a hearty meal of fireflies, whose light was shining through the belly with considerable intensity.

Infection with luminous bacteria is especially liable to occur in any dead marine animal. The flesh is an excellent culture medium. I have seen non-luminous species of squid, recently killed, covered with minute growing colonies, quite evenly spaced, so as to closely resemble luminous species whose light is restricted to scattered light organs over the surface of the body.

Indeed Pierantoni (1918) has carried this idea to extremes. He believes that in the luminous organs of fireflies, cephalopods and Pyrosoma, luminous symbiotic

bacteria occur which are responsible for the light of these animals, and he claims in the case of cephalopods and Pyrosoma to have been able to isolate these in pure culture on artificial culture media. In the firefly they can be seen but not grown and in luminous animals where no visible bacteria-like structures are apparent he believes we are dealing with ultra-microscopic luminous bacteria similar to the pathogenic forms suspected in filterable viruses. While the assumption of ultra-microscopic organisms makes the refutation of Pierantoni's views a somewhat hazardous task, no one can deny that even an ultra-microscopic organism will be killed by boiling with 20 per cent. (by wt.) HCl for 6 hours. As we shall see, the luminous material of Cypridina, an ostracod crustacean, can withstand such prolonged boiling with strong acid. The light of one animal at least, and I believe many others also, cannot be due to any sort of symbiotic organism.

Apart from these cases where light is actually produced but is not primary, not produced by the animal itself, there are many forms whose surface is so constituted as to produce interference colors. This is true in many cases among the birds and butterflies whose feathers and scales are iridescent. Some of these have been erroneously described as luminous. Perhaps the best known case among aquatic animals is Sapphirina, a marine copepod living at the surface of the sea, and especially likely to be collected with other luminous forms. Its cuticle is so ruled with fine lines as to diffract the light and flash on moving much as a fire opal. Needless to say no trace of light is given off from this animal in a totally dark room.

It has often been supposed that the eye of a cat or of other animals is luminous. The eyes of a moth, also,

can be seen to glow like beads of fire when it is flying about a flame. Both of these cases are, however, purely reflection phenomena and due to reflection out of the eye again of light which has entered from some external source. The correct explanation was given by Prevost in 1810. The eye of any animal is quite invisible in absolute darkness. The same explanation applies to the moss, Schistostega, which lives in dimly illuminated places and whose cells are almost spherical, constructed like a lens, so as to refract the light and condense it on the chloroplasts at the bottom of the cells. Some of this light is reflected out of the cells again and gives the appearance of self-luminosity. The alga, Chromophyton rosanoffii, is another example of apparent luminosity, due to reflection from almost spherical cells.

There are several light phenomena known which have nothing to do with living organisms. Commonest of these is St. Elmo's fire ("corposants" of English sailors), a glow accompanying a slow brush discharge of electricity, which appears as a tip of light on masts of ships, spires of churches or even the fingers of the hand. It is best seen in winter during and after snowstorms and is a purely electrical phenomenon.

Less well known is the Ignis fatuus (Will-o'-the-Wisp, Jack-o'-Lantern, spunkie), a fire seen over marshes and stagnant pools, appearing as a pale bluish flame which may be fixed or move, steady or intermittent. So uncommon is this phenomenon that its nature is not well understood, but it is believed to be the result of burning phosphine (PH₃ + P₂H₄), a self-inflammable gas, generated in some way from the decomposition of organic matter in the swamp. The difficulty with this explanation is that

phosphine is not known as a decomposition product of organized matter. Methane (CH₄), a well-known decomposition product of organic matter and abundantly formed in swamps, will burn with a pale bluish flame and some have thought the Ignis fatuus to be the result of this gas. As methane is not self-inflammable there remains the difficulty of explaining how it becomes lighted. Although still a mystery, it is possible that this light is also of electrical origin or that in some cases large clusters of luminous fungi have been observed.

The flashing of flowers, especially those of a red or orange color, like the poppy, which many observers have noticed during twilight hours, is a purely subjective phenomenon due to the formation of after images in eyes partially adapted to the dark. This flashing, first observed by the daughter of Linnæus, is never observed in total darkness or in the direct field of vision, but only in the indirect field as during a sidelong glance at the plant.

There are some cases of luminosity on record in connection with man himself. (See Heller, 1854). Before the days of aseptic and antiseptic surgery, wounds frequently became infected with luminous bacteria and glowed at night. The older surgeons even supposed that luminous wounds were more apt to heal properly than non-luminous ones. We know that luminous bacteria are non-pathogenic, harmless organisms and the presence of these forms even on dead fish or flesh never accompanies but always precedes putrefaction. As recorded by Robert Boyle, no harm has come from eating luminous meat, unless it may also have become infected with pathogenic forms.

A few cases of luminous individuals have been noted

in which the skin was the source of light, especially if the person sweated freely. It is possible that here we are again dealing with luminous bacteria upon the accumulations of substances passed out in the sweat, which serves as a nutrient medium.

There are also on record, in the older literature, cases of luminous urine, where the urine when freshly voided was luminous. If these observations are correct and they may, perhaps, be doubted, we are at present uncertain of the cause of the light. Bacterial infections of the bladder are not inconceivable although luminous bacteria are strongly aerobic and would not thrive under anaerobic conditions. I can state from my own experiments that luminous bacteria will live in normal human urine, but not well. In albuminous urines it is very likely that they would live better, and it is possible that the luminous urines reported are the results of luminous bacterial infection. On the other hand, the light may be purely chemical, due to the oxidation of some compound, an abnormal incompletely oxidized product of metabolism, which oxidizes spontaneously in the air. We know that sometimes these errors in metabolism occur, as in alkaptonuria, where homogentistic acid is excreted in the urine and on contact with the air quickly oxidizes to a dark brown substance. Light, however, has never been reported to accompany the oxidation of homogentistic acid, although it does accompany the oxidation of some other organic compounds. (See Chapter II.)

Finally, we may inquire to what extent luminous animals may be utilized by man. Leaving out of account the use of tropical fireflies for adornment by the natives of the West Indies and South America and the use for bait,

in fishing, of the luminous organ of a fish, Photoblepharon, by the Banda islanders, we find that luminous bacteria are of value for certain purposes in the laboratory.

These methods are all due to Beijerinck (1889, 1902). He has, for instance, used luminous bacteria for testing bacterial filters. If there is a crack in the filter the bacteria will pass through and a luminous filtrate is the result, but a perfect filter allows no organisms to pass and gives a dark filtrate.

Luminous bacteria are also very sensitive to oxygen and cease to luminesce in its absence. By mixing luminous bacteria with an emulsion of chloroplasts (from clover leaves) in the dark, allowing the bacteria to use up all the oxygen, and then exposing the mixture to light of various colors, the effect of different wave-lengths in causing photosynthesis could be studied. Only if the chloroplasts are exposed to a color in the spectrum which decomposes CO₂ with liberation of oxygen do the bacteria luminesce, and when this oxygen is used up by the bacteria, the tube again becomes dark. Beijerinck has also worked out a method of testing for maltose and diastase with luminous bacteria, based on the fact that a certain form, Photobacterium phosphorescens, will only produce light in presence of maltose or diastase which will form maltose from starch.

Although Dubois and Molisch have both prepared "bacterial lamps" and although it has been suggested that this method of illumination might be of value in powder magazines where any sort of flame is too dangerous, it seems doubtful, to say the least, whether luminous bacteria can ever be used for illumination. Other forms, perhaps, might be utilized, but bacteria produce too weak

a light for any practical purposes. The history of Science teaches that it is well never to say that anything is impossible. It is very unlikely that any luminous animal can be utilized for practical illumination, but there is no reason why we cannot learn the method of the firefly. Then we may, perhaps, go one step further and develop a really efficient light along similar lines. To what extent our inquiry into the "secret of the firefly" has been successful may be gleaned from the following pages.

CHAPTER II
LUMINESCENCE AND INCANDESCENCE

Modern physical theory supposes that light is a succession of wave pulses in the ether caused by vibrating electrons. The light to which we are most accustomed—sunlight, electric light, gaslight, etc.,—is due to electrical phenomena connected more or less directly with the high temperature of the source of the light. Every solid body above the temperature of absolute zero is giving off waves of different wave-length (λ) and frequency (ν) but of the same velocity (υ), in vacuo, 180,000 miles, or 300,000 kilometres a second. In fact, υ (a constant)=λ_ν, so that it is only necessary to designate the wave-length in order to characterize the waves. This is radiant energy or radiant flux.

As everyone knows, the long waves given off in largest amount from objects at comparatively low temperatures give the sensation of warmth. As we raise the temperature, in addition to these longer heat waves, those of shorter and shorter wave-length are given off in sufficient quantity to be detected. At 525° C., rays of about λ=.76µ in length are just visible as a faint red glow to the eye. As the temperature increases still shorter wave-lengths become apparent, and the light changes to dark red (700°), cherry red (900°), dark yellow (1100°), bright yellow (1200°), white-hot (1300°) and blue-white (1400° and above). Above λ=.4µ the waves again fail to affect our eye, and, although they are very active in producing chemical changes, we have no sense organs for perceiving

them. Thus, a white-hot object liberates radiant energy or flux of many different wave-lengths corresponding to what we know as "heat, light and actinic rays." All can be dispersed by prisms of one or another appropriate material to form a wide continuous spectrum, such as that indicated in [Fig. 1]. Radiant energy of λ=.76µ to λ=.4µ, evaluated according to its capacity to produce the sensation of light, is spoken of as visible radiation or luminous flux.

Below the infra-red comes a region of wave-length as yet uninvestigated, and beyond this may be placed the Hertzian electric waves of long wave-length used in wireless telegraphy. Above the ultra-violet comes another region as yet uninvestigated, and then Röntgen rays (X-rays) and radium rays, of exceedingly short wave-length. These last types need not concern us except in that we may later inquire if they are given off by luminous animals. The shortest of the ultra-violet are known as Schumann and Lyman rays. These relations are brought out in [Table 2].

TABLE 2.
Wave-lengths of Various Kinds of Radiation

Wave-lengths of light are usually given in Ångstrom units. One micron (µ)=.001 mm.=1000 millimicrons (µµ)=10,000 Ångstrom units (Å) or tenth metres=10^-10 metres or 10^-8 centimetres. The entire scale of wave-lengths extends from 10⁶ to 10^-9 centimetres.

Fig. 1.—Schematic representation of various types of radiation to form a wide continuous spectrum.

The total radiant energy which a body emits is a function of its temperature and for a perfect radiator, or what is known as a black body, the total radiation varies as the fourth power of the absolute temperature, T. (Stefan-Boltzmann Law). The radiant energy emitted at different wave-lengths is not the same but more energy is emitted at one particular wave-length (λ_max.) than at longer or shorter ones, depending also on the temperature. If the various waves are intercepted in some way, their relative energy can be measured by an appropriate instrument and spectral energy curves can be drawn, showing the distribution of energy throughout the spectrum. [Fig. 2] gives a few of the curves, and it will be noted that the maximum shifts toward the shorter waves the higher the temperature. In fact, for a black body λ_max.×T=2890, and at 5000° C. (about the temperature of the sun) λ_max. lies within the visible spectrum. In gas or electric lights it lies in the infra-red region. The area enclosed by these spectral energy curves represents the total energy emitted, and, knowing this and the area enclosed by the curve of visible radiation, it is easy to determine how efficient a source of light is as a light-producing body. We shall inquire more fully into this question in Chapter III, in considering the efficiency of the firefly as a source of light.

Fig. 2.—Distribution of energy throughout the spectrum of the sun, electric arc, and gas light (after Nichols and Franklin). Ordinates show the relative intensities of different wave-lengths emitted. The notches in the curve represent absorption bands and the dotted line represents what the radiation from the sun would be if no selective absorption occurred. V=violet and R=red end of visible spectrum. (Courtesy Macmillan Co.)

A body which emits light because of its (high) temperature is said to be incandescent and we speak of temperature radiation. We know, however, of many cases where substances give off light at temperatures much below 525° C. They do not follow the Stefan-Boltzmann law. The light emission is stimulated by some other means than heat. Such bodies we speak of as luminescent, and in this category belong all luminous animals. The distinction between light and luminescence was first pointed out by Wiedemann (1888). It is usual to classify luminescences, according to the means of exciting the light, into the following groups:

• Thermoluminescence
• Phosphorescence and Fluorescence
• Photoluminescence
• Cathodoluminescence
• Anodoluminescence
• Radioluminescence
• Triboluminescence and Piezoluminescence
• Crystalloluminescence
• Chemiluminescence

The luminescence which appears in a vacuum tube when an electric current is passed through it is sometimes spoken of as electroluminescence. As electroluminescence and also thermoluminescence are really special cases of phosphorescence or fluorescence and tribo-and crystalloluminescence are closely allied, the classification has only the merit of emphasizing the means of producing light. Let us examine each kind in turn in order that we may place the light of animals, organoluminescence or bioluminescence (or biophotogenesis), in one of these classes. All are examples of "cold light," light produced at temperature far below those observed in incandescent solids. In this category should be placed also the light from salts in the bunsen flame, for flame spectra and line spectra in general, while only obtained at relatively high temperatures, are not to be confused with the purely temperature radiation from the incandescent particles of carbon in a gas or candle light. The sodium or lithium flame, etc., is not a simple function of temperature and has been spoken of as a luminescence, pyroluminescence. As the luminescence of organisms could in no manner be regarded as a pyroluminescence, occurring at temperatures far above those compatible with life, a consideration of this form of luminescence will be omitted. Some other low temperature flames are known, such as that of CS₂ in air, rich in ultra-violet rays, despite its relatively low temperature. While these are of interest to the physicist and chemist, they can have no direct bearing on the luminescence of animals and their consideration will also be omitted. (See Bancroft and Weiser, 1914-1915.)

Thermoluminescence.—Some substances begin to emit light of shorter wave-length than red, well below 525°.

This is thermoluminescence. Diamond, marble, and fluorite are examples. Only certain varieties of fluorite show the phenomenon well. A crystal of one of these varieties heated in the bunsen flame on an iron spoon will give off a white light long before any trace of redness appears in the iron. Other crystals may luminesce in hot water. In all, this luminescence is dependent on a previous illumination or radiation of the crystal. If kept in the dark for a long time no trace of light appears when fluorite is placed at a temperature of 100°, but after a short exposure to the light of an incandescent bulb, although no light can be observed in the fluorite at room temperature, quite a bright glow appears at 100°. Calcium, barium, strontium, magnesium and other sulphates containing traces of manganese sulphate, show a similar phenomenon after exposure to cathode rays (Wiedemann and Schmidt, 1895 b). They emit light during bombardment, but this soon ceases when the rays are cut off. If the sulphates are now heated they give off light, red in the case of MgSO₄ + MnSO₄, green in the case of CaSO₄ + MnSO₄. The power to emit light on heating may be retained for months after the exposure to cathode rays. The emission of light by bodies after previous illumination or radiation is called phosphorescence and will be considered below. It would seem that the cases of thermoluminescence with which we are acquainted are really cases of phosphorescence intensified by rise of temperature. The spectrum of thermoluminescent bodies, also, is similar to that of phosphorescent ones. (See [Fig. 3].) However, not all phosphorescent materials are also thermoluminescent. The production of light by animals is quite another phenomenon from thermoluminescence.

Phosphorescence and Fluorescence.—Although the word phosphorescence has been used in a very loose way to indicate all kinds of luminescence, and particularly that of phosphorus or of luminous animals, to the physicist it has a very definite meaning, namely, the absorption of radiant energy by substances which afterwards give this off as light. Phosphorescence does not strictly apply to the light of white phosphorus. If the radiant energy is light (visible or ultra-violet) we speak of photoluminescence, if cathode rays we have cathodoluminescence, if anode rays, anodoluminescence, and if X-rays (Röntgen rays) we have radioluminescence. Inasmuch as the α, β, and γ rays of radium correspond to the anode, cathode, and X-rays, respectively, radium radiation also produces luminescence in many kinds of material. If the material gives off the light only during the time it is radiated we speak of fluorescence; if the light persists we speak of phosphorescence. The distinction is perhaps a purely arbitrary one, as there are a great many substances which give off light for only a fraction of a second (1/5000 sec. in some cases) after being illuminated (photoluminescence). Some substances also, which fluoresce at ordinary temperatures, will phosphoresce at low temperatures. Phosphorescence is exhibited chiefly by solids, fluorescence also by liquids and vapors.

Special means must be used to observe a phosphorescence of short duration. E. Becquerel has devised an apparatus for doing this, a phosphoroscope. It consists of revolving disks with holes in them between which the object to be examined is placed. The holes are so arranged that the object is first illuminated and then completely cut off from light. The observer looking at it through

another hole sees it at the moment it is not illuminated and can thus tell if it is phosphorescing. By determining the rate of revolution of the disks it is easy to calculate how long the phosphorescence persists.

While relatively few solids phosphoresce after exposure to light at ordinary temperature a large number of these acquire the property at the temperature of liquid air. Included in the list are such biological products as urea, salicylic acid, starch, glue and egg shells. The temperature also affects the wave-length and hence the color of the light given off. Usually the higher the temperature the shorter the wave-length, but in the case of some bodies (SrS) the wave-lengths become longer at the higher temperature.

The best known cases of phosphorescence which occur at room temperature and the group to which the word phosphorescence is commonly applied, are those of the alkaline earth sulphides (BaS, CaS, SrS) and ZnS. An Italian, Vicenzo Cascariolo, is said to have discovered the Bologna stone (BaSO₄) which, by calcination with charcoal, gave an impure phosphorescent BaS or lapis solaris. Canton's phosphorus (CaS) was later prepared "by heating a mixture of three parts of sifted calcined oyster shells with one part of sulphur to an intense heat for one hour." Hulme spoke of it as the "light magnet of Canton," because of its power of attracting and absorbing light. The pure sulphides do not show this property. Only if small amounts of some other metal such as Cu, Pb, Ag, Zn, Sb, Ni, Bi, or Mn are present, will the sulphide phosphoresce. One part of impurity in a million is often sufficient. Such mixtures, together with a flux of Na₂SO₄,

Li₃(PO₄)₂ or some other fusible salt constitute a "phosphor." A "phosphor" is in reality an example of a solid solution and is the basis of some kinds of luminous paints.

The intensity and duration of a phosphorescent light depend chiefly on the nature of the exciting rays, the color chiefly on the impurity present but the alkaline earth metal also exerts an influence. Rise in temperature increases the intensity but diminishes the duration, so that the total amount of light emitted is about constant at different temperatures.

The spectrum of most phosphorescent substances is made up of one or more continuous bands having maxima at different wave-lengths. In the light incident on a phosphorescent substance are also bands of light rays which are absorbed and whose wave-lengths are more efficient than others in stimulating phosphorescence. These bands in the phosphorescent light are usually of longer wave-length than those in the light which excites the phosphorescence. This fact is known as Stokes' Law, but it has been found not to be universally true. Curiously enough, red and infra-red rays have the power of annulling phosphorescence after a momentary increase in brightness and phosphorescing materials have been used to determine if infra-red rays are given off in the light of the firefly. Ives (1910) showed that infra-red radiation had no power of quenching the light of the firefly as it does the phosphorescent light of Sidot blende (ZnS), one fact tending to show that the firefly's light is not due to phosphorescence. [Fig. 3] is a reproduction of a photograph of the phosphorescence spectrum of ZnS.

Fig. 3. Spectrum of zinc sulphide phosphorescence (after Ives and Luckiesh). Photographs were taken by a special device one minute (middle) and fifteen minutes (bottom) after exposure to the light of the mercury arc and compared with a helium spectrum (top). In the middle photograph, the mercury exciting lines are visible. It will be noted that the narrow band of phosphorescent light does not shift its position during decay of phosphorescence.

Other facts show that the light of luminous animals is in no sense a phosphorescence and is quite independent of previous illumination of the animal. Luminous bac

teria will continue to luminesce although they are grown in the dark for many weeks. Indeed strong light has a bactericidal action on these forms similar to that with ordinary bacteria. With some marine forms light has an inhibiting effect. They lose their power of luminescence during the day and only regain it at dusk or when kept in the dark for some time. Indeed, ordinary light never has the effect of causing luminescence in the same sense as it causes phosphorescence of CaS.

Fluorescence is most efficiently excited by the cathode rays of a vacuum tube. They not only cause the residual gas in the tube to glow (electroluminescence) by which their path may be followed with the eye, but also a vivid fluorescence of the glass walls of the tube, yellow green with sodium glass, blue green with lead and lithium glass. LiCl₂ in the path of cathode rays gives off a blue light; in the path of anode rays a red light; NaCl a blue cathodoluminescence and a yellow anodoluminescence. The spectrum of the latter is a line spectrum of Li or Na, showing the characteristic red or yellow lines similar to those observed where Li or Na is held in the bunsen flame. The spectrum of the salts under excitation of cathode rays is a short continuous one in the blue region. Fluorescent spectra in general are of this nature, made up of short bands of light in one or more regions.

Diamonds, rubies and many minerals fluoresce brilliantly in the path of cathode rays. Some specimens of fluorite (CaF₂) show the phenomenon especially well, whence the name fluorescence. Fluorescent screens of barium platinocyanide, willemite (Zn₂SiO₄), Sidot blend (ZnS) or Scheelite (Ca tungstate) are frequently employed to render visible X-rays. The luminous paint most

used at the present time is ZnS containing a trace of radium salt. The rays of the radium continually emitted cause a steady fluorescence of the ZnS. Indeed, if one examines the paint on the hands of a watch with a lens the flash of light from the impact of alpha particles on the ZnS can be distinctly seen, as in the spinthariscope.

Some animal tissues and fluids, especially the lens of the eye, will luminesce in the path of radium rays, as shown by the experiments of Exner (1903), but there is no evidence that luminous animals are especially active in this respect. Ultra-violet rays have the same action.

The luminous material of practically all luminous forms, if dessicated sufficiently rapidly, can be obtained in the form of a dry powder which will give off light when moistened with water. Coblentz (1912) has exposed this dry material to light, to the ultra-violet spark, and to X-rays and in no case has a phosphorescence or fluorescence ever been observed. I have examined the action of radium upon Cypridina light. There was no intensifying or diminishing effect of twenty milligrams of radium (probably the bromide) on a luminous solution of Cypridina material, nor was phosphorescence or fluorescence excited in a non-luminous extract of the animal. We must conclude that animal light is not a fluorescence of any substance due to radiation produced by the animals themselves.

Many solutions show fluorescence in strong lights. This is especially marked in quinine sulphate, mineral oils, eosin, fluorescein, esculin, rhodamin, chlorophyll, etc. The fluorescence of eosin in 10^-8 grams per cubic centimetre is visible in daylight and 10^-15 grams per cubic centimetre in the beam from an arc lamp. It is difficult to realize that the

bluish fluorescence of quinine sulphate is really an emission rather than a reflection of light. But a test tube of quinine sulphate solution held in the ultra-violet region of a spectrum will glow with a pale blue light, although it is not illuminated with any rays that are visible to our eyes. Concerning this, Stokes, to whom the word fluorescence and much of our knowledge of the subject is due, says, "It was certainly a curious sight to see the tube" (containing quinine sulphate solution) "instantaneously lighted up when plunged into the invisible rays; it was literally 'darkness visible.'" Quinine sulphate absorbs the ultra-violet converting these rays into visible blue ones. Its spectrum is a short continuous one. Most fluorescent substances convert short into longer wave-lengths (Stokes' Law), but some may cause the reverse change.

A substance, fluorescent in solution, has been found in a few luminous animals, notably in several species of fireflies and also in a non-luminous beetle. It is called pyrophorine or luciferesceine. Dubois (1886) has ascribed to pyrophorine the power of absorbing invisible rays and transforming them into visible ones, thus increasing the animal's light. That this is not the case has been shown by the work of Coblentz (1909). He photographed the spectrum of the firefly's light and the fluorescent spectrum of luciferesceine. The latter is almost complementary to the former (see [Fig. 4]) and no trace of the fluorescent spectrum appears in the spectrum of the light of the firefly. McDermott (1911 a) has studied the properties of luciferesceine and regards it merely as an incidental material found in many animals of the Lampyridæ (in some non-luminous forms) and having no connection with

the light production. A trace of alkali usually increases and acid inhibits the fluorescence of solutions.

Fig. 4.—Spectrum of fluorescent substance found in fireflies below (2) and of firefly luminescence above (2) compared with helium vacuum tube (1) (after Coblentz).

Triboluminescence and Piezoluminescence.—Under this head are grouped a number of light phenomena which at first sight may appear to be electrical in nature but in reality are not. The light is produced by shaking, rubbing, or crushing crystals, and only crystalline bodies appear to show triboluminescence or piezoluminescence. A striking case is that of uranium nitrate. Gentle agitation of the crystals is sufficient to give off sparks of light which much resemble the scintillations of dinoflagellates when sea-water containing these animals is agitated. If Romberg's phosphorus, which is fused CaCl₂, is rubbed on the sleeve, it glows with a greenish light. Lumps of cane sugar rubbed together will glow. Saccharin crystals will also light if shaken and Pope (1899) found that the bluish light of saccharin was bright enough to be visible in a room in daytime. It only appeared from impure crystals and freshly crystallized specimens. Other crystals, also, have been found to lose their power of lighting after a time.

Among biological substances, cane sugar, milk sugar, mannite, hippuric acid, asparagin, r-tartaric acid, l-malic acid, vanillin, cocaine, atropin, benzoic acid, and many others show triboluminescence. A long list is given by Tschugaeff (1901), by Trautz (1905), and by Gernez (1905). The spectrum is a short continuous one, the waves emitted depending on the kind of crystal. Thus the color of the light varies among different santonin derivatives from yellow to green. In saccharin it is blue.

Although the light produced by some living organisms resembles triboluminescence in that it may be evoked by

rubbing or shaking the animals, it is in reality fundamentally different since it is dependent on the presence of oxygen whereas triboluminescence is not.

Crystalloluminescence.—Crystalloluminescence is observed when solutions crystallize. It was described by Bandrowski (1894, 1895) in arsenious oxide, in NaF, or if HCl or alcohol is added to hot saturated NaCl solution. A bluish light with sparkling points appeared. All well authenticated cases are exhibited by simple inorganic salts and these are also all triboluminescent. The reverse is not true, however; many triboluminescent substances are not crystalloluminescent. Crystalloluminescence is much less widespread than triboluminescence. Trautz (1905) has studied the matter in a number of compounds and comes to the conclusion that the light is really a special case of triboluminescence in which the growth of individual crystals causes them to rub together. The light becomes much brighter on stirring a mass of crystals which exhibit crystalloluminescence. While in some cases crystalloluminescence is unquestionably due to the triboluminescence of crystals rubbing against each other it is not in every case, as has been clearly shown by the work of Weiser (1918 b). He studied luminescence of saturated aqueous alkali halide solutions (NaCl, KCl, etc.,) upon addition of alcohol or of HCl. The salt crystallizes out under these conditions and Weiser found that the light is brightest when the conditions of concentration of alcohol or of HCl are such as to cause heaping up of Na and Cl ions. He believes that the bluish light which appears is due to the combination of ions in the reaction, Na⁺ + Cl^- = NaCl. Only if this proceeds rapidly enough does luminescence occur. Weiser studied also the crystallo

luminescence and triboluminescence of AsCl₃ and of K₂SO₄. By photographing the luminescence through color screens of different absorptive power (Weiser, 1918, a) a spectrum of the light could be obtained, and it was found to be identical in both the tribo- and crystalloluminescent light; in the case of AsCl₃, a band in the green-blue, blue and violet. Weiser believes the light in this case also to come from recombination of the ions, As⁺⁺⁺ + 3Cl^- = AsCl₃, and that crystalloluminescence in general is due to rapid reformation of molecules from ions broken up by electrolytic dissociation while triboluminescence is due to rapid reformation of molecules from ions broken up by violent disruption of the crystal. Of course in triboluminescent organic crystals which do not dissociate into ions, some other reaction must be responsible for the light. One thing seems certain, that the two types of luminescence are similar. As Bigelow[1] remarks, "It is altogether probable that the cause of this" (crystalloluminescence) "whatever it may be, is the same as the cause of triboluminescence, whatever that may be."

[1] Theoretical and Physical Chemistry, 1912, p. 516.

Crystals are not found in the luminous organs of animals with the exception of the fireflies. In these a layer of cells occurs (see Chapter IV) filled with minute crystals of one of the purine bodies (xanthin or uric acid). One might surmise that the light of the animal was a crystalloluminescence accompanying the formation of these crystals. It is easy to show, however, that the light comes not from the crystal layer but from another layer of cells containing large granules. It is also dependent on the presence of oxygen while crystalloluminescence takes place in the absence of oxygen. The crystal layer possibly

serves as a reflector. Its significance will be discussed in a later chapter.

Fig. 5.—Dubois's figures showing transformation of photogenic granules to crystals (after Dubois).

The light of luminous organisms is quite generally associated with granules. In one of the centipedes (Orya barbarica), which produces a luminous secretion, Dubois (1893) has described the transformation of these granules into crystals and at one time he supposed the light to be a crystalloluminescence. He later reversed this opinion and, certainly, examination of his drawings which are reproduced in [Fig. 5] does not convince one of the actuality of crystal formation.

The phenomenon of lyoluminescence, described by

Wiedemann and Schmidt (1895) as a light accompanying the solution of colored (from exposure to cathode rays) crystals of Li, Na, or K chlorides, is probably due to a triboluminescence from stirring of the crystals during solution.

Chemiluminescence.—As the name implies, chemiluminescence is the production of light during a chemical reaction at low temperatures. This does not mean that the other types of luminescence are not connected with chemical reactions—using the word reaction in a broad sense—for we have reason to believe that in some cases spectra are not characteristic of the element as such but are rather characteristic of a particular reaction in which the element takes part (dissociation into ions, changes from monovalent to bivalent condition, etc.) and that this is the reason one element may show various spectra under different conditions (Bancroft, 1913). The chemiluminescences are rather oxidation reactions involving the absorption of gaseous or dissolved oxygen and may be very easily distinguished from all the previously mentioned luminescences by this criterion. They should, perhaps, more properly be called oxyluminescences.

The glow of phosphorus is the best known case, recognized since phosphorus was first prepared by Brandt in 1669. It is interesting to note that when first prepared phosphorus was regarded as a peculiarly persistent type of phosphor, i.e., a material akin to the impure alkaline earth sulphides.

Fresh cut surfaces of Na and K metal will glow in the dark for some time, especially if warmed to 60°-70° (Linnemann, 1858). A film of oxide is formed over the surface, showing definitely that oxidation has occurred.

Ozone oxidizes organic matter with an accompanying glow (Fahrig, 1890; Otto, 1896). The light from ozone acting on pyrogallol solution is especially bright under certain conditions.

Radziszewski (1877, 1880) gives a long list of substances, chiefly essential oils, which luminesce if slowly oxidized in alcoholic solutions of alkalis. Formaldehyde, dioxymethylen, paraldehyde, metaldehyde, acroleïn, disacryl, aldehydeammonia, acrylammonia, hydrobenzamid, lophin, hydroanisamid, anisidin, hydrocuminamid, hydrocinamid, besides waxes, and such biological substances as glucose, lecithin, cholesterin, cholic, taurocholic, and glycocholic acids, and cerebrin, all luminesce on oxidation. Radziszewski himself and many other authors have compared the light of organisms to this type of luminescence. Indeed the incorrect identification of granules found in the cells of practically all luminous tissues as oil droplets, is largely due to the influence of Radziszewski's work. Dubois (1901 b) has added esculin, and Trautz (1904-5) many aldehydes and phenol derivatives, including vanillin, papaverin, tannic and gallic acids, besides glycerol and mannite to the list of biological substances oxidizing with light production. Guinchant (1905) has described oxyluminescence of uric acid and asparagine, Weitlaner (1911) of substances in humus and McDermott (1913) of substances in urine and the anaerobic alkaline hydrolysis products of glue and Witte's peptone. Pyrogallol is especially prone to luminesce, as was first noticed by Lenard and Wolf (1888) in developing a photographic plate with pyrogallol developer. Later the luminescence was studied in some detail by Trautz and Schloringin (1904-5) who developed the well-known luminescent mix

ture of pyrogallol, formaldehyde, K₂CO₃ and H₂O₂. As I have shown, pyrogallol can be oxidized in a great many different ways, and some of these are of great interest, for they very closely imitate the mechanism for the production of light in organisms. These are recorded in [Table 3], which also includes various other types of oxyluminescence of general or biological interest.

TABLE 3
Types of Oxyluminescent Reactions

• 1. Oxidation in air spontaneously.
• (a) At ordinary temperatures. [Phosphorus. Fresh-cut surfaces of Na or K. Thiophosgene and Thio-ethers (RCS.OR).]
• (b) At melting or vaporizing points. (Fats, terpenes, sugars, resins, gums, ether, silk and others.)
• 2. Oxidation in aqueous or alcoholic alkalies. (Many organic substances.)
• 3. Oxidation in hypoiodites, hypobromites, or hypochlorites. (Many organic substances.)
• 4. Oxidation in peroxides (H₂O₂ or Na₂O₂). (Many organic substances.)
• 5. Oxidation in ozone. (Many organic substances.)
• 6. Oxidation in acid permanganate. (Pyrogallol.)
• 7. Oxidation in persulfates and perborates. (Formaldehyde, paraformaldehyde.)
• 8. Oxidation in perchlorates, periodates, and perbromates. (Palmitic acid.)
• 9. Combination of 2 and 4. (Many organic substances.)
• 10. Combination of 3 and 4. (Many organic substances.)
• 11. Oxidation with H₂O₂ and hæmoglobin or vegetable oxidases. (Pyrogallol, gallic acid, lophin, esculin.)
• 12. Oxidation with H₂O₂ and MnO₂, Fe₂Fe(CN)₆ Mn(OH)₂ + Mn(OH)₃ Ag₂O, chromium oxide, cobalt oxide. (Pyrogallol.)
• 13. Oxidation with H₂O₂ and ferrocyanides, chromates, bichromates, permanganates, Fe salts, and Cr salts. (Pyrogallol, esculin.)
• 14. Oxidation with H₂O₂ and collodial Ag. Pt. Pd. Au. (Pyrogallol.)

The spectrum of chemiluminescent reactions has been described in a few instances as continuous but no definite measurements of its extent have been made. Radziszew

ski (1880) found the light of lophin oxidized in alcoholic caustic alkali, examined with a two-prism spectroscope, to give a continuous spectrum, brightest at E, with the red and violet ends lacking. Trautz (1905, p. 101) states that the pyrogallol-formaldehyde-Na₂CO₃-H₂O₂ reaction gives a continuous spectrum from the red to the blue green with maximum brightness in the orange. Weiser (1918 a) has studied the spectra of some chemiluminescent reactions by photographing the light behind a series of color screens. He finds also that the spectra are short, with maximum intensity in various regions. Thus, amarin oxidized by chlorine or bromine, extends from the yellow to greenish blue with a maximum in the green while phosphorus, dissolved in glacial acetic acid and oxidized with H₂O₂, luminesces from yellow green to violet.

The spectra of luminous animals are quite similar to those of chemiluminescent reactions. Moreover, as we have seen, chemiluminescence is essentially an oxyluminescence, since oxygen is necessary for the reaction. All luminous animals also require oxygen for light production. Therefore, bioluminescence and chemiluminescence are similar phenomena and they differ from all the other forms of luminescence which we have considered. The light from luminous animals is due to the oxidation of some substance produced in their cells, and when we can write the structural formula of this photogenic substance and tell how the oxidation proceeds, the problem of light production in animals will be solved.

CHAPTER III
PHYSICAL NATURE OF ANIMAL LIGHT

Interest in the light of animals from a physical standpoint has centred around questions of quality, efficiency and intensity, but in only one group of luminous animals, the beetles, have accurate measurements of these characteristics been made. This is due in part to the abundance of these forms and their appeal to human interest and in part because they are among the brightest of luminous organisms. Weak lights are not only difficult to measure but, when dispersed to form spectra, give bands so faint that their limits are very difficult to see and more so to photograph. Very few organisms produce light visible to the fully light-adapted eye. Although their light may seem quite bright to the dark-adapted eye, the dark-adapted eye is a poor judge of the quality, i.e., the color of a light. This is because of the Purkinje phenomenon, a change in the region of maximum sensibility of the retina with change in intensity of the light. For an equal energy spectrum, to the normal, completely light-adapted eye, yellow-green light of wave-length, λ = .565µ, appears the brightest, but when the light is made fainter the maximum shifts first to the green and then to the blue. The dark-adapted eye can see green or blue better than yellow and for this reason weak lights will appear more green or blue than stronger ones of the same energy distribution. Also two weak lights of the same spectral composition may appear different in color if they differ much in intensity. This is illustrated in [Fig. 6].

Fig. 6.—Visibility curves for three illuminations showing the shift in region of maximum visibility, or Purkinje phenomenon (after Nutting).

The shift in sensibility of the eye occurs in illuminations of between 0.5 and 50 metre-candles and represents a change from central cone vision (high intensities) to peripheral rod vision (low intensities). The fovea centralis lacks rods and this part of the eye becomes practically color blind at very low intensities of light. Below 0.5 and above 50 metre-candles visibility varies but little with change in intensity. It is clearly necessary then to distinguish between the physical objective phenomenon of light and the physiological subjective sensation of light.

It is a fact that different luminous animals produce light of quite different colors as judged by our eye. A range of spectral tints has been described which extends from red to violet but "yellowish," "greenish" and "bluish" tints are commonest. Indeed one or two animals possess several luminous organs emitting lights of different colors. This is true in a South American firefly, Phengodes,

whose lights are red and greenish yellow, and in the deep sea squid, Thaumatolampas diadema, which produces lights of three colors, two shades of blue and red. The red light in the case of the squid appears to be due to a red color screen formed by the chromatophores, but in Phengodes no screen is present.

TABLE 4
Wave-lengths of Fraunhofer Lines and Prominent Lines in Line Spectra

FRAUNHOFER LINES

BUNSEN FLAME LINES

PLÜCKER TUBE LINES

As we have seen, difference in color of the light does not necessarily indicate difference in spectral composition because of the Purkinje effect. However, examination of the spectrum of various luminous forms has very clearly indicated that the different colors are really due to light rays of different wave-length and are not the result of any subjective phenomena. To facilitate comparison, spectral lines and colors are given in [Table 4]. The first adequate observations on the spectra of luminous animals were made by Pasteur (1864), who studied Pyrophorus and found a continuous spectrum unbroken by light or dark bands. Lankester (1868) discovered a similar continuous spectrum in Chætopterus insignis and placed its limits from line 5 to 10 on Sorby's Scale (about λ = 0.55µ

to λ = 0.44µ). Young (1870) first recorded the limits of the firefly spectrum as a little above C (λ = .6563µ) to F (λ = .4861µ). Since then a number of luminous forms have been examined and all are found to give short continuous spectra (not crossed by light or dark bands or lines) lying in different color regions. Thus, Conroy (1882) examined the glowworm (Lampyris noctiluca) light and observed a band extending from λ = 0.518µ to λ = 0.656µ. Dubois (1886) states that the spectrum of Pyrophorus noctilucus, the West Indian "Cucullo," extends from slightly further than the Fraunhofer B line to the F line, while Langley and Very (1890), working on the same form, placed the limits at λ = 0.468µ to

λ = 0.640µ. It consists, then, of a broad band chiefly in the green and yellow. But, "would the light not extend farther were it bright enough to be seen?... if the light of the insect were as bright as that of the sun would it not extend equally far on either side of the spectrum?" "It is impossible to increase the intrinsic brilliancy by any optical device, but if it be impossible to make the light of the insect as bright as that of the sun, it is on the other hand quite possible to make the light of the sun no brighter than that of the insect ..." Langley and Very investigated this question, forming a solar spectrum from sunlight of the same intensity as that of Pyrophorus and a Pyrophorus spectrum together in the same field of the spectroscope. The latter was very much shorter than the solar spectrum, showing that its length was not due to weakness of the red and blue rays but to their absence. Later Ives and Coblentz (1910) photographed the spectrum of a firefly (Photinus pyralis), together with that of a carbon glow lamp, on plates sensitive to all wave-lengths of visible rays under conditions which would have recorded all visible radiations given off. They found the spectrum to extend only from λ = 0.51µ to λ = 0.67µ ([Fig. 7]). Another species of firefly (Photuris pennsylvanica) was found by Coblentz (1912) to give a spectrum extending from λ = 0.51µ to λ = 0.59µ ([Fig. 8]). The Photinus light extends much further into the red and it is easy to distinguish between Photinus and Photuris in nature, merely by the reddish tint of the light of the former. These photographic records show conclusively that the color of the light of luminous animals is not a subjective phenomenon due to the Purkinje effect and the low intensity of the light, but is real, an actual difference in spec

tral composition of the light emitted. Neither is it due, at least in the fireflies examined, to the existence of color screens which absorb certain rays, allowing only those of a definite color to pass. The spectra of forms thus far investigated are reproduced in [Fig. 9] and recorded in [Table 5]. It will be noted that they vary considerably in position but are all of the same type. The spectrum of Cypridina hilgendorfii is the longest thus far investigated (λ = .610µ to λ = .415µ), extending well into the blue, and the light of this form is very blue in appearance.

Fig. 7.—Spectra of carbon glow lamp, A, firefly (Photinus pyralis); B, and helium vacuum tube, C (after Ives and Coblentz).

Fig. 8.—Spectra of helium vacuum tube (1); carbon glow lamp (2); the firefly, Photinus pyralis (3); and the firefly Photuris pennsylvanica (4) (after Coblentz).

Fig. 9.—Spectra of various luminous animals (after McDermott). 1. Portion of the visible solar (grating) spectrum showing Fraunhofer lines. 2. Pyrophorus noctilucus (Langley and Very.) 3. Lampyris noctiluca (Conroy). 4. Photinus pyralis (Ives and Coblentz). 5. Photinus consanguineus (Coblentz). 6. Photuris pennsylvanica (Coblentz). 7. Phengodes laticollis (McDermott). 8. Bacterium phosphoreum, B. phosphorescens or Bacillus photogenus (Molish). 9. Photobacterium indicum (Barnard). 10. Mycelium X (Molish). 11. Luminous bacteria (Förster). 12. Agaricus sp.? (Ludwig). 13. Fluorescent spectrum of luciferesceine of Photinus pyralis (Coblentz). Only the extreme ends of the bands are shown and no attempt is made to indicate the relative density of different portions of the spectra.

Table 5.—Limits of Spectra of Various Luminous Organisms

As first shown by Dubois (1886) for Pyrophorus, and confirmed by myself for Cypridina, the light is not polarized in any way. I may add that the Cypridina light like any other light may be polarized by passing through a Nicol prism.

Several writers [Dubois (1914 book)], Fischer (1888), Molisch (1904 book) have noticed that the light of luminous bacteria changes in color if grown on different culture media. Light which is "silver white" on dead fish becomes "greenish" on salt-peptone-gelatin media and more yellow on salt-poor media. Peron (1804) and Panceri (1872) describe the light of Pyrosoma as yellow to greenish after death of the animal and reddish on stimulation; then fading out through orange, yellow, greenish and azure blue. Polimanti (1911) describes the normal light of Pyrosoma as greenish, and states that as the animals die, or if they are kept at temperatures above the optimum, the light becomes more red. McDermott (1911, b) noticed that the light of fireflies placed in liquid air became decidedly reddish just before going out and on rewarming the first light to appear was reddish followed by the proper shade at higher temperatures. I have frequently observed

a more reddish color from luminous tissues of the firefly upon the addition of coagulants such as alcohol, and have noted that the light of Cypridina becomes weaker and more yellow at both low (0°) and high (50°) temperatures. The meaning of these color changes will be discussed in [Chapter VII].

The efficiency of any light may be defined in several different ways: (1) By the percentage of visible wave-lengths in the total amount of radiation emitted, i.e., visible radiation divided by total (heat, visible, actinic) radiation; (2) by considering, in addition to visible radiation ÷ total radiation, the sensibility of the eye to different wave-lengths, visible radiation × visual sensibility ÷ total radiation. Visible radiation × visual sensibility is spoken of as luminosity; (3) by the amount of light (expressed in candles) produced in relation to a given expenditure of energy or in relation to the cost of the energy expended. Thus, of the radiation emitted from an incandescent electric lamp only a small per cent. is light, the rest being heat and actinic rays. It is therefore very far from being 100 per cent. efficient. If there were no infra-red or ultra-violet in the radiation from an incandescent lamp its efficiency would be 100 per cent. if we disregarded visual sensibility. But if we take into account the fact that the eye is most sensitive to yellow green, a source of light, even though emitting only visible radiation, would not be 100 per cent. efficient unless its maximum of emission corresponded also with the maximum of visual sensibility. We shall return to this question in a later paragraph. Looking at the question from the standpoint of energy consumption, the carbon incandescent lamp gives one mean spherical candle for 4.83 watts (watt = 10⁷ ergs

per sec.), while the tungsten lamp gives one mean spherical candle for 1.6 watts, about one-third the energy, and the latter is consequently more efficient.

As we know practically nothing of the energy transformations occurring during the process of light production in organisms, all statements regarding the efficiency of their light are based on relations between the visible radiation and total radiation. This involves a measurement of rays in the infra-red region (heat rays) and ultra-violet region (actinic rays) as well as the light rays proper, and any other radiant energy produced. While all spectroscopic investigations show that the spectrum of luminous animals never extends to the limits of the visible spectrum in either the red or violet, it is possible that bands occur in the infra-red or ultra-violet, and special methods must be employed to detect these. Radiations of all kinds, if converted into heat on striking the blackened surface of a thermopile, bolometer, or radiometer can be measured by changes in temperature and the relative amounts of energy represented be compared in a common unit, the calorie. By proper screening, all rays except the visible light rays can be cut off from the measuring instrument and the amounts of energy represented in light and in total radiation thus be determined.

Dubois (1886) first studied this problem in Pyrophorus by the use of a thermopile and galvanometer and found a small amount of radiation from the luminous region in excess of that from a non-luminous region. It amounted to a galvanometer deflection of 0.95° and was increased 0.3° during the flash of the insect on electrical stimulation. This increase of 0.3° is possibly due to heat produced on muscular contraction. In any case the amount of heat

radiated in comparison with that of the candle is very small indeed. A more careful study has been made by Langley and Very (1890) with the bolometer. They point out first of all that the total radiation from the most powerful luminous organ (the abdominal one) of Pyrophorus which affected their bolometer slightly, would, in the same time (10 seconds), be sufficient to raise the temperature of an ordinary mercurial thermometer having a bulb 1 cm. in diameter by rather less than 2.3 × 10^-6° C. We may thus gain some idea of the magnitude of the measurements to be made. The radiation from Pyrophorus which affected their bolometer was shown to be due merely to the "body heat"[2] of the insect, and it is largely cut off by a plate of glass which is opaque to all wave-lengths of 3µ or more. These waves are given off by bodies at temperatures below 50° C. and belong "to quite another spectral region to that in which the invisible heat associated with light mainly appears." Langley and Very then compared the radiation from a non-luminous bunsen flame and the Pyrophorus light, interposing a plate of glass in each case to cut off the waves longer than 3µ, and found several hundred times more radiation in the case of the bunsen burner but, nevertheless, perceptible radiation from Pyrophorus. The former consisted of radiant heat shorter than λ = 3µ and extending up to the visible light rays (λ = 0.7µ since the bunsen flame emitted no light). The very slight effect of the Pyrophorus radiation must be due to wave-lengths between λ = 3µ and λ = 0.468µ, the limit of the Pyrophorus spectrum in the blue. Langley and Very assumed it to be due entirely to the band of

visible light, λ = 0.640µ to λ = 0.468µ, and assumed that no invisible heat rays were produced. All of the energy of Pyrophorus light would therefore lie in the visible region and its efficiency (light rays ÷ heat + light + actinic rays) would be 100 per cent. Later, Langley (1902) reinvestigated the radiation of Pyrophorus and could detect no heating whatever with the bolometer. "A portion of the flame of a standard sperm candle, equal in area to the bright part of the insects, gave under the same circumstances, a bolometric effect of such magnitude that had the heat of the insect been 1/80,000 as great as that from the candle, it would certainly have been recognized." Coblentz (1912) also, using a vacuum thermopile of Pt and Bi, was unable to detect any infra-red radiation from Photinus pyralis, but found that the temperature of this firefly is slightly lower than the air. These temperature measurements will be discussed in a later chapter.

[2] Langley and Very evidently supposed that the body temperature of the firefly, like the mammal or bird, is higher than its surroundings.

The assumption of Langley and Very that the small amount of Pyrophorus radiation passing glass is all light has been called into question by Ives (1910), who points out that Langley and Very failed to use a screen which would cut off either the visible rays or the invisible rays between 3µ and 0.7µ. They really left the question open as to whether the effect of Pyrophorus light on their bolometer was due to the visible band of rays or to this plus another band in the infra-red. "The firefly's actual efficiency as a light source is dependent to a large degree on the radiation being confined to the visible region. If there should be found infra-red of quantity comparable to the visible, the firefly, while still a very efficient source would not be, as usually supposed, the example of an ideally efficient light produced by nature."

Ives investigated the question further by the phosphor-photographic method. "In brief it consists of this: Phosphorescence, which is excited in various substances by exposure to short waves (blue, violet or ultra-violet), is destroyed by exposure to longer waves (orange, red, infra-red). Thus, a surface of Balmain's paint or of Sidot blende, excited to phosphorescence and then exposed in a spectrograph, will have areas of reduced brightness wherever long-wave energy has fallen upon it. If this surface is then laid on a photographic plate for a short period, a permanent record is obtained on the plate after development." Preliminary tests showed that the method was applicable in the case of weak light such as the firefly spectrum and also if the light is intermittent like the firefly. With Sidot blend (ZnS) the extinguishing action extends from λ = 0.6µ to λ = 1.5µ. A sheet of deep ruby glass, which cut off all the visible rays of the firefly but allowed infra-red to pass, was placed between the firefly light and a surface of phosphorescent Sidot blend which was exposed to the firefly flashes for three and a half hours. No extinction of phosphorescence occurred, while without the ruby glass, extinction, due to the orange rays of the visible firefly light was noticeable in 20 minutes. There is thus no infra-red of an intensity at all comparable to the visible as far as λ = 1.5µ, the lower limit of the phosphor-photographic method. Coblentz (1912) had examined the transparency of the dry chitinous integument of various fireflies ([Fig. 10]) in the infra-red and reports it to be fairly transparent down to λ = 2.8µ, opaque between λ = 2.8µ and λ = 3.8µ, transparent again to λ = 6µ, and opaque beyond that. The infra-red could, then, if it were emitted, largely pass through the integument which

is similar in absorption properties to complex carbohydrates. Transparency of the integument to the ultra-violet was not studied.

Fig. 10.—Transmissivity of the integument of fireflies to infra-red radiation (after Coblentz.)

Although photographs of the spectrum of firefly (Photinus) light show that it extends only to the beginning of the blue, Forsyth (1910) reports ultra-violet radiation in luminous bacteria. He exposed a plate for 48 hours to the spectrum of bacterial light dispersed by a quartz prism and got a continuous band from λ = 0.50µ (the lower limit of sensitivity of the plate) to λ = 0.35µ. However, McDermott (1911 d) was unable to observe fluorescence of p-amino-ortho-sulpho-benzoic acid, which responds to the ultra-violet light. Molisch (1904, book) photographed bacterial and fungus light through glass and through a piece of quartz and found no difference in density on the plate. As the exposure was brief, to avoid saturation, and as the ultra-violet, which passes quartz but not glass, has a much

greater action on the plate than visible light, we must conclude that ultra-violet is absent. Ives (1910) investigated the spectrum of Photinus pyralis, using a quartz spectroscope, and found no evidence of ultra-violet radiation, at least as far as λ = 0.216µ.

It will thus be seen that the radiation from the firefly has been very carefully studied and that no waves are given off from λ = 1.5µ to λ = 0.216µ with the exception of the short band (λ = 0.67µ to λ = 0.51µ) in the visible, and it is highly probable that no radiation is given off with wave-lengths longer than λ = 1.5µ. The firefly light remains, then, 100 per cent. efficient, differing from all our artificial sources of light, the best of which does not approach this value. As Langley and Very express it in the title to their paper, it is "the cheapest form of light," not cheapest in the sense of that we can reproduce it commercially at less cost than other lights, but cheaper in the sense that it is the most economical in the energy radiated. This energy is all light and no heat. "Cold light" has actually been developed by the firefly and concerning which "we know of nothing to prevent our successfully imitating."

Fig. 11.—Spectral energy curves of various fireflies and the carbon glow lamp (after Coblentz).

I have already pointed out that we may also consider the efficiency of a light in relation to the sensibility of our own eye. That is, we take into account not only the energy distribution in the spectrum of the light but also the fact that different wave-lengths of an equal energy spectrum affect our eye very differently. As the normal light-adapted eye is most sensitive to yellow green of λ = 0.565µ, monochromatic light of this wave-length will appear much brighter than monochromatic light of any other wave-length with the same energy. Monochromatic

light of λ = 0.565µ will then be the theoretically most efficient possible, when we consider the energy radiated in relation to the sensitivity of our eye. This is the usual method of determining the luminous efficiency of artificial

lights and is obtained from a knowledge of the radiated energy and the visual sensibility. Reduced luminous efficiency = light (radiated energy × visual sensibility) or luminosity ÷ total radiated energy.

Fig. 12.—Visibility curves of various investigators obtained by different methods (after Hyde, Forsyth and Cady).

Fig. 13.—Luminous efficiency of the 4-watt carbon glow lamp, shaded area ÷ total area (after Ives and Coblentz).

Fig. 14.—Luminous efficiency of the firefly, shaded area ÷ total area (after Ives and Coblentz).

The spectral energy curve for the firefly has been worked out by Ives and Coblentz (1910), using a photographic method in which the intensities of different wave-lengths of the firefly (Photinus pyralis) light is com

pared with that of a carbon glow lamp by measuring the amount of photochemical change produced on panchromatic photographic plates. [Fig. 11] gives the energy curves of various fireflies and the carbon glow lamp in the same spectral region. The visual sensibility curve used by Ives and Coblentz is that of Nutting (1908, 1911), based on Konig's data. It is reproduced in [Fig. 6]. The latest visibility curve is that of Hyde, Forsyth and Cady (1918), reproduced in [Fig. 12]. It is based on observations of twenty-nine individuals. As individuals vary considerably in their sensibility to different wave-lengths, the visibility curve represents an average, but it is the only standard we have with which to evaluate the energy we call light. Color-blind individuals would have a visibility curve very different from normal individuals. Composite curves showing the luminous efficiency of the 4-watt carbon glow lamp and the firefly, both in relation to visibility, are given in Figs. [13] and [14], respectively. In these figures

the luminous efficiency is the shaded area ÷ total area, 0.43 per cent. for the carbon glow lamp and 99.5 per cent. for the firefly, "these numbers representing the relative amounts of light (measured on a photometer) for equal amounts of radiated energy—a striking illustration of the wastefulness of artificial methods of light production. From the specific consumption of the tungsten lamp (1.6 watts per spherical candle) and the mercury arc (.55 watts per spherical candle) we obtained by comparison with the carbon filament that their luminous efficiencies are 1.3 and 3.8 per cent. The most efficient artificial illuminant therefore has about 4 per cent. of the luminous efficiency of the

firefly." This is calculated to be .02 watts per candle. More recent determinations (Coblentz, 1912), using a new sensibility curve of Nutting's (1911) for a partially light-adapted eye, give the reduced luminous efficiency as 87 per cent. for Photinus pyralis, 80 per cent. for Photinus consanguineus and 92 per cent. for Photuris pennsylvanica.

Fig. 15.—Spectral energy, luminosity and visibility curves (after Gibson and McNicholas)

• A. Spectral energy curve of Hefner lamp.
• B. Spectral energy curve of acetylene flame.
• C. Spectral energy curve of tungsten (gas-filled) glow lamp.
• D. Spectral energy curve of black body at 5000° absolute (sunlight).
• E. Spectral energy curve of blue sky.
• Hg. Spectral energy curve of Heræus quartz mercury lamp.
• Lv. Visibility curve for human eye.
• La. Luminosity of Hefner lamp.
• Le. Luminosity of blue sky.

The luminous efficiencies of various forms of artificial illuminants have been calculated by Ives (1915) and are given together with that of the firefly in [Table 6]. [Fig. 15]

gives spectral energy curves for various illuminants reduced to 100 at λ = .590µ, luminosity curves for the Hefner lamp and blue sky, and a visibility curve worked out by Coblentz and Emerson (1917) from observations on 130 individuals.

Table 6
Luminous Efficiencies of Various Illuminants

The firefly light by the above method of calculating efficiency is not 100 per cent. efficient because its maximum (λ = 0.567µ) does not correspond with the maximum sensibility of the eye (λ = 0.565µ), but taking into consideration also other effects of color, the firefly light would be a still more inefficient and trying one for artificial illumination, as all objects would appear a nearly uniform

green hue. Indeed the distortion would be even greater than with the mercury arc, whose objectionable green hue is so well known. "We may say, therefore, that the firefly has carried the striving for efficiency too far to be acceptable to human use; it has produced the most efficient light known, as far as amount of light for expenditure of energy is concerned, but has produced it at the (inevitable) expense of range of color. The most efficient light for human use, taking into account both color and energy-light relationships, would be a light similar to the firefly light containing no radiation beyond the visible spectrum, but differing from it by being white." (Ives, 1910.) Although the spectral energy curve for Cypridina light has not been worked out, it will be noted that the Cypridina spectrum is much longer than that of the firefly, more nearly approaching the spectrum of an incandescent solid giving white light. It approaches, but does not attain the ideal.

Although Muraoka (1896) and Singh and Maulik (1911) have described radiations coming from fireflies which would pass opaque objects and affect a photographic plate, and Dubois reports the same from bacteria, the existence of such radiation has been denied by Suchsland (1898), Schurig (1901) and Molisch (1904 book). The experiments of Molisch on luminous bacteria are of greatest interest, for they are very carefully controlled and show without a doubt that black paper or Zn, Al, or Cu sheet will allow no rays from these organisms to pass that will affect a photographic plate, even after several days' exposure. The visible light of luminous bacteria will affect the plate after one second exposure. Moreover, Molisch has pointed out the errors of those who claim to

have found penetrating radiation in luminous forms. It seems that certain kinds of cardboard, especially yellow varieties, or wood, will give off vapors that affect the photographic plate. The action is especially marked with damp cardboard at a temperature of 25°-35° C., and Dubois and Muraoka must have used such cardboard to cover their plates. A piece of old dry section of beech or oak trunk, placed on a photographic plate for 15 hours in a totally dark place, will register a beautiful picture of the annual rings of growth, medullary rays, junction of bark and wood, etc. Russell (1897) had previously found that many bodies, both metals and substances of organic origin (gums, wood, paper, etc.), placed in contact with photographic plates, would affect them, and concluded that vapors and not rays were the active agents. As a dry piece of wood has a very definite smell, there is something given off which can affect our nose and there is no reason why it should not change, by purely chemical action, the photographic plate. This action of wood on the plate is prevented by interposing a sheet of glass. Frankland (1898) has described similar vapors coming from colonies of Bacillus proteus vulgaris and B. coli communis which affect a photographic plate laid directly over the colonies in an open petri dish. There is no effect if the glass cover of the petri dish is between plate and bacteria. There is, then, no specific emission of X-rays or similar penetrating radiation from luminous tissues which will affect the photographic plate through opaque screens.