The Nature of Animal Light Part 4

Bacteria G to F extending Barnard, Photographic.

toward D for 1902 long exposure Bacteria Somewhat beyond Fisher, Eye observation.

G to D 1888 Bacteria .58 - .43 Forster, Eye observation 1887 Zeiss. Abbe microspectral ocular.

Bacteria >.500 to .350 Bright Forsyth, Photographic, band 1910 quartz at .4 spectroscope.

Agarious 0.56-0.48 Ludwig, Eye observation, melleus (approximately) 1884 Sorby Brown microspectroscope.

Xylaria .54 - .46 Ludwig, Eye observation, hypoxylon (approximately) 1884 Sorby Brown microspectroscope.

Micrococcus b into the Ludwig, Eye observation, Pflugeri violet 1884 Sorby Brown microspectroscope.

Mycelium X .570 - .480 Molish, Eye observation, 1904, Zeiss comparison book spectroscope.

Bacterium .570 - .450 Molish, Eye observation, phosph.o.r.eum 1904, Zeiss comparison book spectroscope.

Bacterium .570 - .450 Molish, Eye observation, phosph.o.r.escens 1904, Zeiss comparison book spectroscope.

Bacillus .570 - .450 Molish, Eye observation, photogenes 1904, Zeiss comparison book spectroscope.

Pseudomonas .570 - .450 Molish, Eye observation, lucifera 1904, Zeiss comparison book 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 pa.s.sing 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 = 10^7 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 gla.s.s which is opaque to all wave-lengths of 3 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 a.s.sociated 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 gla.s.s in each case to cut off the waves longer than 3, 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 ? = 3 and extending up to the visible light rays (? = 0.7 since the bunsen flame emitted no light). The very slight effect of the _Pyrophorus_ radiation must be due to wave-lengths between ? = 3 and ? = 0.468, the limit of the _Pyrophorus_ spectrum in the blue. Langley and Very a.s.sumed it to be due entirely to the band of visible light, ? = 0.640 to ? = 0.468, and a.s.sumed 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 circ.u.mstances, 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 a.s.sumption of Langley and Very that the small amount of _Pyrophorus_ radiation pa.s.sing gla.s.s 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 3 and 0.7. 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 quant.i.ty 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: Phosph.o.r.escence, 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 phosph.o.r.escence 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 ? = 0.6 to ? = 1.5. A sheet of deep ruby gla.s.s, which cut off all the visible rays of the firefly but allowed infra-red to pa.s.s, was placed between the firefly light and a surface of phosph.o.r.escent Sidot blend which was exposed to the firefly flashes for three and a half hours. No extinction of phosph.o.r.escence occurred, while without the ruby gla.s.s, 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 ? = 1.5, 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 ? = 2.8, opaque between ? = 2.8 and ? = 3.8, transparent again to ? = 6, and opaque beyond that. The infra-red could, then, if it were emitted, largely pa.s.s through the integument which is similar in absorption properties to complex carbohydrates.

Transparency of the integument to the ultra-violet was not studied.

[Ill.u.s.tration: 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 ? = 0.50 (the lower limit of sensitivity of the plate) to ? = 0.35. 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 gla.s.s 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 pa.s.ses quartz but not gla.s.s, 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 ? = 0.216.

It will thus be seen that the radiation from the firefly has been very carefully studied and that no waves are given off from ? = 1.5 to ? = 0.216 with the exception of the short band (? = 0.67 to ? = 0.51) in the visible, and it is highly probable that no radiation is given off with wave-lengths longer than ? = 1.5. 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 t.i.tle 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."

[Ill.u.s.tration: 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 ? = 0.565, 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 ?

= 0.565 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.

[Ill.u.s.tration: FIG. 12.--Visibility curves of various investigators obtained by different methods (_after Hyde, Forsyth and Cady_).]

[Ill.u.s.tration: FIG. 13.--Luminous efficiency of the 4-watt carbon glow lamp, shaded area total area (_after Ives and Coblentz_).]

[Ill.u.s.tration: 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 compared with that of a carbon glow lamp by measuring theamount 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 ill.u.s.tration 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_.

[Ill.u.s.tration: 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.

H_g_. Spectral energy curve of Heraeus quartz mercury lamp.

L_v_. Visibility curve for human eye.

L_a_. Luminosity of Hefner lamp.

L_e_. 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 ? = .590, 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_

------------------------+------------------------+--------+---------------- Efficiency Illuminant and Commercial Lumens (visible commercial description rating per radiation watt visual sensibility total radiation) ------------------------+------------------------+--------+---------------- Carbon incandescent lamp 4 watts per mean 2.6 0.0042 oval anch.o.r.ed (treated) horiz. c. filament Tungsten incandescent 1.25 watts per mean 8.0 .013 lamp, vacuum type horiz. c. Mazda, type c 600 C. P. 20 amp., 19.6 .032 0.5 w. p. c. Series type C. Carbon arc (open) 9.6 amp. clear globe 11.8 .019 Open arc, yellow flame, 10 amp. D. C. 44.7 .072 inclined trim Quartz mercury arc 174-197 volt, 4.2 amp. 42.0 .068 Gla.s.s mercury arc 40-70 volt, 3.5 amp. 23.0 .037 Nernst lamp 4.8 .0077 Acetylene 1 L per hr. consumption .67 .0011 Petroleum lamp .26 .0004 Open flame gas burner Bray 6 high pressure .22 .00036 Incandescent gas lamp, .350 lumens per 1.2 .0019 low pressure B. T. U. per hr. Incandescent gas lamp, .578 lumens per 2.0 .0032 high pressure B. T. U. per hr. Firefly 629.0 .96 ------------------------+------------------------+--------+----------------

The firefly light by the above method of calculating efficiency is not 100 per cent. efficient because its maximum (? = 0.567) does not correspond with the maximum sensibility of the eye (? = 0.565), 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 pa.s.s 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 pa.s.s 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 gla.s.s.

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 gla.s.s 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.

A similar conclusion is reached if we attack the problem in another way.

X-rays and radium rays (Becquerel rays) cause fluorescence of ZnS, barium platinocyanide, willemite (Zn_{2}SiO_{4}), and calcium tungstate. Coblentz (1912) showed that the firefly will cause no fluorescence of a barium platinocyanide screen and I have been unable to detect fluorescence of zinc sulphide, barium platinocyanide, zinc silicate (willemite) or calcium tungstate shielded from _Cypridina_ light by black paper, although the light of this organism is quite bright enough to cause phosph.o.r.escence of zinc sulphide without the black paper. The samples of the above four substances all showed fluorescence in presence of radium rays, but only the ZnS phosph.o.r.esces after exposure to light rays, although the willemite was phosph.o.r.escent after exposure to the ultra-violet.

While photometry at low intensities is a difficult procedure at best, if the light varies in intensity or is a flash, accurate measurements become well-nigh impossible. The figures given for intensity of animal luminescence must, therefore, be accepted with a realization of the difficulties of measurement. By candle is meant the international candle, unless otherwise specified, equal to 1.11 Hefner candles (H. K.) 0.1 pentane lamp and 0.104 carcel units. It is a measure of intensity.

Amount of light, or light flux, measured in lumens, is that emitted in a unit solid angle (area/_r_^2) by a point source of one candle-power. One candle-power emits 4p lumens. The latest figure for the mechanical equivalent of light at ? = .566 is .0015 watt (Hyde, Forsyth and Cady, 1919), _i.e._, 1 lumen = .0015 watt. One watt is 10^7 ergs (one joule) per second.

The illumination (of a surface) is that given by one candle at one metre, the candle metre (C.M.) or lux. The surface then receives one lumen per square metre. A metre kerze (M.K.) is the illumination given by one Hefner candle at one metre distance.

The brightness of a surface is measured in lamberts or millilamberts. A lambert is "the brightness of a perfectly diffusing surface radiating or reflecting one lumen per square cm." A millilambert is 1/1000 lambert.

For further definitions the reader is referred to the reports of the committee on nomenclature of the Illuminating Engineering Society.

Dubois (1886) states that one of the prothoracic organs of _Pyrophorus noctilucus_ has a light intensity of 1/150 Ph[oe]nix candle of eight to the pound (probably about equivalent to 1/150 candle) and that 37 or 38 beetles (each using all three light organs) would produce light equivalent to one Ph[oe]nix candle. Langley (1890) found that to the eye the prothoracic organ of _Pyrophorus noctilucus_ gave one-eighth as much light as an equal area of a candle and the actual candle-power of the insect was 1/1600 candle. It may be remarked in pa.s.sing how widely divergent these observations are.

For the flash of the firefly (_Photinus pyralis_) Coblentz (1912) found variation from 1/50 to 1/400 candle, the predominating values being around 1/400 candle. A continuous steady glow is sometimes obtained from this insect and it proved to be of the order of 1/50,000 candle.