424 PHYSIOLOGICAL LIGHT. 



filially ultramarine blue. Certain Appendicular ia liave a tricolored 

 luminosity. 



Most of tliese variations depend upon corresponding modifications 

 of tlie internal photogenic mechanism; so when the light of Noctiluca 

 changes from blue to white by reason of fatigue or the somatic death 

 of the animal, the protoplasmic granulations and the beams that they 

 emit become at once smaller and more numerous. But the coloration 

 of the light may also depend upon circumstances independent of its 

 mode of production, the color of the integument, or of the blood. For 

 example, by injecting eosin into the blood of Pyroplwrus the light 

 changes from its usual green color to rose. 



Finally, the pale blue tint seen in many marine animals, larvse, mush- 

 rooms, and bacteria, must in certain cases be entirely attributed to the 

 slight intensity of the luminosity. For the same reason, a spectroscopic 

 examination does not enable us to distinguish the color of the different 

 rays that compose their spectra, but the extreme limits of such spectra, 

 fixed by various observers, leave no doubt as to their poly chromatic 

 nature. The luminous intensity is generally only slightly increased 

 in the middle regions of this pale spectrum. 



The light of insects furnishes, on the contrary, a fine continuous 

 spectrum, without bands or lines, but in which the various component 

 rays can be readily distinguished. That of Pyroplwrus noctilucus, for 

 example, is quite remarkable when the animal is very luminous; some- 

 what extended on the side of the red, it reaches as far as the first blue 

 rays and covers about twenty-four divisions of the micrometer. Its 

 approximate limits are on one side the line B, on the other the line F, of 

 the solar spectrum. So the middle part of that spectrum corresponds 

 to the maximum luminous intensity of the spectrum of Pyroplwrtis. 



It should also be noted that the impression produced on the eye warns 

 us that the composition of this light is not the same as that from arti- 

 ficial sources. It is easy to assure ourselves that this is the case by 

 arranging an artificial diaphragm so as to give to a beam of artificial light 

 a photometric intensity approximately equal to that of the luminous 

 organ and then comparing its dispersion with that of the rays from 

 Pyroplwrus. The graphic representation of the results obtained by this 

 spectroscopic experiment may serve to give an idea of the respective 

 composition of the light of these various sources, but this method is 

 evidently extremely imperfect, since we can not by ordinary photomet- 

 ric processes compare the intensity of lights of different composition. 



A spectro -photometric analysis, however, taking the wave-lengths for 

 abscissae and the intensities of the luminous rays for ordinates, shows 

 that the area between the axis of the wave-length and the curve is, in 

 the light from Pyroplwrus, almost wholly occupied by green and yellow 

 rays (fig. 1). The maximum of intensity corresponds to a wave-length 

 ji 0.528,56. Now, this wave-length is precisely the same as that which 

 affords the maximum of clearness in the solar spectrum, while in the 

 flame of a candle the maximum of luminous intensity corresponds to 



