The Nature of Animal Light Part 3

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 H_{2}O_{2} and haemoglobin or vegetable oxidases.

(Pyrogallol, gallic acid, lophin, esculin.)

12. Oxidation with H_{2}O_{2} and MnO_{2}, Fe_{2}Fe(CN)_{6} Mn(OH)_{2} + Mn(OH)_{3} Ag_{2}O, chromium oxide, cobalt oxide. (Pyrogallol.)

13. Oxidation with H_{2}O_{2} and ferrocyanides, chromates, bichromates, permanganates, Fe salts, and Cr salts. (Pyrogallol, esculin.)

14. Oxidation with H_{2}O_{2} 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. Radziszewski (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-Na_{2}CO_{3}-H_{2}O_{2} 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 H_{2}O_{2}, 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, ? = .565, 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 ill.u.s.trated in Fig. 6.

[Ill.u.s.tration: 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, _PhenG.o.des_, 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 chromatoph.o.r.es, but in _PhenG.o.des_ no screen is present.

TABLE 4

_Wave-lengths of Fraunhofer Lines and Prominent Lines in Line Spectra_

FRAUNHOFER LINES

======================================================================== Line Color Wave-lengths Source ( = /1000) ------------------+------+---------------------+------------------------ A Red 759.4 (band) Oxygen in atmosphere.

a Red 718.5 (band) Water vapor atmosphere.

B Red 686.7 Oxygen vapor atmosphere.

C Red 656.3 Hydrogen in sun.

D_{1} D_{2} Yellow 589.6, 589.0 Sodium in sun.

E Green 527.0 Calcium in sun.

b_{1} b_{2} b_{4} Green 518.4, 517.3, 516.8 Magnesium in sun.

F Blue 486.1 Hydrogen in sun.

G Violet 430.8 Calcium in sun.

H K Violet 396.9, 393.4 Calcium in sun.

BUNSEN FLAME LINES

=============================================== Source Color Wave-lengths ( = /1000) ----------+--------+--------------------------- Pota.s.sium Red 769.9, 766.5 (double) Lithium Red 670.8 Sodium Yellow 589.6, 589.0 (double) Thallium Green 535.1 Magnesium Green 518.4 Strontium Blue 460.7 ----------+--------+---------------------------

PLuCKER TUBE LINES

=============================================== Source Color Wave-lengths ( = /1000) ----------+--------+--------------------------- Mercury Yellow 579.0, 576.9 Green 546.1 Blue 491.6, 435.8 Violet 407.8, 404.7 Hydrogen Red 656.3 Blue 486.1, 434.1 Helium Red 728.2, 706.5, 667.8 Yellow 587.6 Green 504.8, 501.6, 492.2 Blue 471.3, 447.2 Violet 438.8, 402.6, 388.8 ----------+--------+---------------------------

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 _Chaetopterus insignis_ and placed its limits from line 5 to 10 on Sorby's Scale (about ? = 0.55 to ? = 0.44). Young (1870) first recorded the limits of the firefly spectrum as a little above _C_ (? = .6563) to _F_ (? = .4861). 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 ? = 0.518 to ? = 0.656. 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 ? = 0.468 to ? = 0.640. 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 ? = 0.51 to ? = 0.67 (Fig. 7). Another species of firefly (_Photuris pennsylvanica_) was found by Coblentz (1912) to give a spectrum extending from ? = 0.51 to ? = 0.59 (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 spectral 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 pa.s.s. 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 (? = .610 to ? = .415), extending well into the blue, and the light of this form is very blue in appearance.

[Ill.u.s.tration: FIG. 7.--Spectra of carbon glow lamp, A, firefly (_Photinus pyralis_); B, and helium vacuum tube, C (_after Ives and Coblentz_).]

[Ill.u.s.tration: 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_).]

[Ill.u.s.tration: 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. _PhenG.o.des laticollis_ (McDermott). 8.

_Bacterium phosph.o.r.eum, B. phosph.o.r.escens or Bacillus photogenus_ (Molish). 9. _Photobacterium indic.u.m_ (Barnard). 10. _Mycelium X_ (Molish). 11. _Luminous bacteria_ (Forster). 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_

============================================================================ Light Spectrum () Emission Observer Method and remarks maximum ----------------+----------------+---------+-----------+-------------------- Cypridina 0.610-0.415 Harvey, Eye observation, hilgendorfii 1919 Zeiss comparison spectroscope.

Chaetopterus 0.55-0.44 Lancaster, Eye observation.

insignis (approximately) 1868 Pyrophorus 0.72-0.486 Dubois, Eye observation.

noctilucus 1886 Pyrophorus .640 - .468 0.57 Langley Eye observation noctilucus and Very, and comparison (thoracic 1890 with solar light) spectrum of equal intensity.

Pyrophorus .663 - .463 noctilucus (abdominal light) Photinus .67 - .51 Ives and Photographic pyralis Coblentz, comparison with 1909 carbon glow lamp of equal intensity.

Photuris .59 - .51 .552 Coblentz, Photographic pennsylvanica 1912 comparison with carbon glow lamp of equal intensity.

Photinus .65 - .52 .578 Coblentz, Photographic consanguineus 1912 comparison with carbon glow lamp of equal intensity.

PhenG.o.des .65 - .52 McDermott, Eye observation.

laticollis 1911 e Lampyris .656- .518 Conroy, Eye observation.

(glow worm) 1910 Photinus .670- .487 Young, Eye observation 1870 direct vision spectroscope.