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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 laboratories 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.