40 



A PHYSICAL STUDY OF THE I^IRUFLrY. 



XI. INFRA-RED ABSORPTION MEASUREMENTS. 



In view of the fact that it has been impossible to detect infra-red radiation 

 emanating from the firefly, the question arose as to whether there is no infra- 

 red radiation, or whether the radiation is absorbed after it leaves the photo- 

 genic cells. Obviously there can be but little infra-red radiation absorbed 

 without special provision to keep the temperature normal, otherwise the 

 insect would become hot. The solution of this question was undertaken 

 from two points of view. Ives* searched for radiation lying between 0.7/i 

 and i.5ju by noting the extinguishing action upon the phosphorescence of 

 Sidot blende when exposed directly to the light of the firefly, and when a 

 plate of red glass was interposed. Exposing the phosphorescing plate 

 directly to the light of the firefly, the intensity of the phosphorescence was 

 markedly reduced in half an hour, but exposing the plate through red glass 

 for 3I hours had no reducing effect upon the intensity of the phosphores- 

 cence of the Sidot blende. Since the reducing effect was practically the 



Fig. 14. — Transmissivity of the integument of fireflies. 



same for the infra-red to 1.5/1 as for the visible, it was concluded that no 

 radiations lying between 0.7/i and i .5/1 were emitted. Using a quartz spec- 

 trograph, no emission could be detected in the ultra-violet. The writer 

 attacked the problemf by examining the opacity of the outer integument 

 (the chitinous layer) of various fireflies for different regions of the infra-red. 

 For, assuming that appreciable radiation is generated in the photogenic 

 cells, it is important to learn what frequencies can pass out into space. 



In Fig. 14, curve a gives the transmission through one of the horny abdom- 

 inal segments of a dried specimen of the Cuban firefly, Pyrophorus noctiluciis. 

 This integument is dark brown (in reflected light), which causes heavy 

 absorption through the visible spectrum to the red, at which point the trans- 

 mission increases rapidly. In this firefly, it may be recalled, the abdominal 

 light is emitted through a slit between two of the segments, of which the 



*Ives, Phys. Rev., 31, p. 637, 1910; Electrical World, 56, Oct. 13, 1910. 

 jRlectrical World, 56, p. 1012, 1910; also 54, p. 1184, 1909. 



