INFRA-RED ABSORPTION MEASUREMENTS. 4 1 



above is a sample. The size of the specimen examined was about 1.5 X 

 7 mm. In this same illustration, Fig. 14, curve b gives the transmission 

 through one of the eye-like fenestrate membranes on the thorax of the Cuban 

 firefly, and through which is emitted the so-called "thoracic light." The 

 specimen was only 1.5X2 mm., and hence very difficult to examine. After 

 removing all the photogenic cell ligaments which adhered to it this specimen 

 was fairly translucent. In the region from im to 2^1 the transmissivity is 

 probably higher than observed, the low values being due to scattering of the 

 radiation in passing through the specimen. 



Curve c, Fig. 14, gives the transmission through the outer integument 

 covering the photogenic cells of Phoiinus pyralis. The specimen examined 

 was a single dried abdominal segment, 1.5 X4 mm., which had been moist- 

 ened in water and freed from all ligaments and photogenic material. The 

 specimen was not very smooth and it was only semi-translucent. No doubt 

 a better sample could have been prepared by dissecting a fresh undried 

 insect. In both curves, b and c, the opacity at i to 2/i and at 5^1 is greatly 

 increased by scattering of the transmitted radiation which did not reach 

 the bolometer. However, as was anticipated in previous communications, 

 these curves have the characteristic absorption spectrum of complex carbo- 

 hydrates, in which there is great (and usually complete) opacity from 2.8/x 

 to 3.8/x, and beyond 6/i, with a fairly transparent region at 4.5^ to 5/x. It 

 may be noticed that these specimens were dry. If they had been fresh, 

 then the water present would have absorbed considerable of the radiation at 

 4 to 5M. 



During the present summer an examination was made of the infra-red 

 absorption of the fresh photogenic substance of Photinus pyralis, excised and 

 placed between fluorite plates. The material was an inhomogeneous white 

 mass, which in the preliminary examination showed complete opacity from 

 3M to 3.8/x, a more transparent region at 4.5 to 5.5M) followed by complete 

 opacity beyond 5.8^. This is the same sort of absorption as found in the 

 chitinous layer, Fig. 14. A thin layer of the white fluorescent fluid which 

 exudes freely from the wings of the pyralis was also examined between fluo- 

 rite plates. Complete opacity was observed from 2.9 to 3. 2/1 and beyond 

 5.8/i; and an appreciable absorption band was observed at 4.7/x. These 

 bands were considered to be due to water, and since no others were observed 

 a more thorough examination seemed unnecessary. Apparently the fluor- 

 escent material has no appreciable absorption bands in this region of the 

 spectrum. 



The experiments of Ives have bridged the gap from the visible to 1.5^- 

 The data then published by the writer showed that but little if any radiation 

 of wave-lengths greater than 2. 5 /Lt would pass outinto spaceeven if generated 

 in the photogenic cells. Thatwhichcan beemitted at 4. 5/x to 5.5juwill beso 

 weak in intensity that it will hardly be distinguished from the emission spec- 

 trum due to the "animal heat" (maximum emission at 8 to lOjj) radiated 

 from the surface of the body. By exclusion, we have, therefore, reduced the 

 unexplored region to that part of the spectrum lying between the wave- 

 lengths 1 .5/i and 2.5/1. There are no types of radiators known which would 

 lead one to expect emission bands in this region of the spectrum of the light 

 of the firefly, and while it is important to be on the alert for the unexpected. 



