CHAPTER III

Chapter 51,881 wordsPublic domain

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, [lambda] = .565 mu, 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.

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

======================================================================== Line |Color | Wave-lengths | Source | | ( mu mu = mu/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

PLUeCKER TUBE LINES

=============================================== Source | Color | Wave-lengths ( mu mu = mu/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 [lambda] = 0.55 mu to [lambda] = 0.44 mu). Young (1870) first recorded the limits of the firefly spectrum as a little above _C_ ([lambda] = .6563 mu) to _F_ ([lambda] = .4861 mu). 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 [lambda] = 0.518 mu to [lambda] = 0.656 mu. 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 [lambda] = 0.468 mu to [lambda] = 0.640 mu. 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 [lambda] = 0.51 mu to [lambda] = 0.67 mu (Fig. 7). Another species of firefly (_Photuris pennsylvanica_) was found by Coblentz (1912) to give a spectrum extending from [lambda] = 0.51 mu to [lambda] = 0.59 mu (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 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 ([lambda] = .610 mu to [lambda] = .415 mu), extending well into the blue, and the light of this form is very blue in appearance.

TABLE 5.--_Limits of Spectra of Various Luminous Organisms_

============================================================================ Light | Spectrum ( mu) |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. Phengodes | .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. 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 | |Foerster, |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, phosphoreum | | | 1904, | Zeiss comparison | | | book | spectroscope. Bacterium | .570 - .450 | |Molish, |Eye observation, phosphorescens| | | 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 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 deg.) and high (50 deg.) temperatures. The meaning of these color changes will be discussed in