Table 5 — Limits of Spectra of Various Luminous Organisms
| Light | Spectrum (µ) | Emission maximum | Observer | Method and remarks |
|---|---|---|---|---|
| Cypridina hilgendorfii | 0.610-0.415 | Harvey, 1919 | Eye observation, Zeiss comparison spectroscope. | |
| Chætopterus insignis | 0.55-0.44 (approximately) | Lancaster, 1868 | Eye observation. | |
| Pyrophorus noctilucus | 0.72-0.486 | Dubois, 1886 | Eye observation. | |
| Pyrophorus noctilucus (thoracic light) | .640 -.468 | 0.57 | Langley and Very, 1890 | Eye observation and comparison with solar spectrum of equal intensity. |
| Pyrophorus noctilucus (abdominal light) | .663 -.463 | |||
| Photinus pyralis | .67 -.51 | .552 | Ives and Coblentz, 1909 | Photographic comparison with carbon glow lamp of equal intensity. |
| Photuris pennsylvanica | .59 -.51 | Coblentz, 1912 | Photographic comparison with carbon glow lamp of equal intensity. | |
| Photinus consanguineus | .65 -.52 | .578 | Coblentz, 1912 | Photographic comparison with carbon glow lamp of equal intensity. |
| Phengodes laticollis | .65 -.52 | McDermott, 1911 e | Eye observation. | |
| Lampyris (glow worm) | .656-.518 | Conroy, 1910 | Eye observation. | |
| Photinus | .670-.487 | Young, 1870 | Eye observation direct vision spectroscope. | |
| Bacteria | G to F extending toward D for long exposure | Barnard, 1902 | Photographic. | |
| Bacteria | Somewhat beyond G to D | Fisher, 1888 | Eye observation. | |
| Bacteria | .58 -.43 | Förster, 1887 | Eye observation Zeiss. Abbe microspectral ocular. | |
| Bacteria | >.500 to.350 | Bright band at.4 | Forsyth, 1910 | Photographic, quartz spectroscope. |
| Agarious melleus | 0.56-0.48 (approximately) | Ludwig, 1884 | Eye observation, Sorby Brown microspectroscope. | |
| Xylaria hypoxylon | .54 -.46 (approximately) | Ludwig, 1884 | Eye observation, Sorby Brown microspectroscope. | |
| Micrococcus Pflugeri | b into the violet | Ludwig, 1884 | Eye observation, Sorby Brown microspectroscope. | |
| Mycelium X | .570 -.480 | Molish, 1904, book | Eye observation, Zeiss comparison spectroscope. | |
| Bacterium phosphoreum | .570 -.450 | Molish, 1904, book | Eye observation, Zeiss comparison spectroscope. | |
| Bacterium phosphorescens | .570 -.450 | Molish, 1904, book | Eye observation, Zeiss comparison spectroscope. | |
| Bacillus photogenes | .570 -.450 | Molish, 1904, book | Eye observation, Zeiss comparison spectroscope. | |
| Pseudomonas lucifera | .570 -.450 | Molish, 1904, book | Eye observation, Zeiss comparison spectroscope. |
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.
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.
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.”
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.
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 compared 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.
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.
Notes
2 Langley and Very evidently supposed that the body temperature of the firefly, like the mammal or bird, is higher than its surroundings.