346 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



For the sake of convenience the subject will be considered in the 

 following sections in tins paper: 



1 . Physical properties of physiologic light. 



2. The chemistry of the photogenic process. 



3. The effect of chemical reagents, etc., on the luminous tissue. 



4. The photogenic organs. 



5. Fluorescent substances in luminous insects. 



6. Biologic relations of the phenomena. 



1. PHYSICAL PROPERTIES OF PHYSIOLOGIC LIGHT. 



It is an interesting and significant fact that the luminous radia- 

 tions of the majority of luminescent organisms produce upon the 

 human retina the sensations of yellowish green, green, or bluish 

 green. That this color is the result of the actual composition of the 

 emitted light and not a subjective phenomenon, has been demon- 

 strated by a number of investigators. 



The light of one of the common fireflies of the eastern United States, 

 Photinus py rolls, has recently been made the subject of a very inter- 

 esting spectrophotographic study by Drs. Ives and Coblentz, ( 31 ) at 

 the Bureau of Standards. These observers found the light of this 

 insect to resemble very closely the light of the Cuban cucuyo (Pyro- 

 phorus noctilucus Linn.), as studied and described by the late Prof. 

 S. P. Langley and Prof. F. W. Very ( 39 ) 20 years ago. Briefly, the 

 spectrum of the light of Photinus pyralis consists of a structureless, 

 unsymmetrical band in the red, yellow, and lower blue portions of the 

 visible spectrum, with a maximum at about that portion having the 

 greatest illuminating effect with the minimum of actinic and thermal 

 effects. It gives no hint of continuation in the infra-red or ultra- 

 violet portions of spectrum. More recently, Ives ( 32 ) has shown that 

 there is no infra-red radiation between 0.7065 micron and 1.5 microns 

 in the light of the firefly, nor is any ultra-violet radiation present. 

 Coblentz ( 5 ) has shown that the chitin covering the luminous organs 

 of the firefly has a very low transmissivity for radiations of greater 

 wave length than 2.5 microns; so low, in fact, that it would be diffi- 

 cult, in these wave lengths, to distinguish radiations from the photo- 

 genic organs from those due to the ordinary animal heat. However, 

 the same author ( 5 *> 6 ) has found that the luminous segments of the 

 firefly do give evidence of being at a slightly higher temperature than 

 the rest of the insect's body, and that the ventral side of the luminous 

 segments is at a higher temperature than the dorsal. He did not 

 observe an increase of temperature during light emission. 



While it seems probable that the light of most other luminous 

 organisms is essentially similar to that of Photinus pyralis, slight dif- 

 ferences may be noted even between closely related species. A few 



