THE FLUORESCENT SUBSTANCE IN FIREMJES. 37 



serious matter. To measure such a temperature difference is 110 small 

 undertaking. To go one step farther and determine how this temperature is 

 modified, as for example by the flash, is a different problem requiring further 

 investigation. 



In Fig. 13, C, is shown a type of (vacuum) bismuth-silver thermopile with 

 which it is purposed to make further radiation measurements. This pile 

 consists of jo junctions of silver wire 0.019 nim. in diameter and bismuth 

 wire 0.04 mm. in diameter, blackened with a composition of platinum black 

 and lamp black. The central junctions are insulated with shellac, placed 

 in contact and backed with a receiver of pure tin, 0.01 mm. thick and 1 mm. 

 in diameter. In air it is one-half as sensitive as a 20-element Rubens linear 

 thermopile. It is about 4 times as sensitive as the radiometer used to 

 measure heat from stars, when used with a galvanometer of the same period 

 as the radiometer. Its resistance is 19.3 ohms. 



X. THE FLUORESCENT SUBSTANCE IN FIREFLIES. 



During the photographic work two years ago, in cleaning the test tubes 

 which had contained fireflies, the writer noticed that the wash water had a 

 peculiar faint bluish tinge, which he considered due to some fluorescent 

 material. In order to demonstrate to others that it was not "blue sky," 

 "decomposition bacteria," etc., an attempt was made to obtain a pure solu- 

 tion and photographs were made of the fluorescent material.* 



The usual procedure to obtain a pure solution was to place the insects in a 

 strong solution of alcohol and water. The albuminous material was precip- 

 itated with lead acetate and the solution was concentrated by boiling. 

 The solution was then neutralized with potassium oxalate to prevent tur- 

 bidity while photographing the fluorescence spectrum, which was obtained 

 with the large spectrograph employed in photographing the light emitted by 

 the firefly. vSuch a photograph of the fluorescent spectrum is shown in 

 Plate 1, C. The cadmium spark produced a greater fluorescence than either 

 zinc or aluminum, and was therefore used as an exciting source. The ultra- 

 violet light tends to produce a cloudiness in the solution which must then be 

 filtered. The fluorescence spectrum shows no structure and seems to be of 

 the same nature as is commonly observed in many substances. The main 

 point of interest is the extent (from 0.38/i to 0.5 i/x) of the fluorescence spec- 

 trum, which is complementary with the light emitted by the firefly. (See 

 the right-ha 11 d side of Plate 1 , C, which gives the spectrum of the light emitted 

 by Photinus.) However, no special significance is attached to this fact. 

 The maximum emission of the fluorescence spectrum lies at about 0.41/^. 

 The absorption in the ultra-violet appears to be very strong, as indicated by 

 the fact that, unless the solution is very dilute, the fluorescence extends but a 

 few millimeters into the solution. When concentrated the solution becomes 

 yellowish and but little fluorescence is noticeable. On dilution, however, 

 the fluorescence reappears. The yellowish color is attributed to the exten- 

 sion of the ultra-violet absorption, of the fluorescent substance, into the 

 visible spectrum on concentrating the solution. The material decomposes 



'"Coblentz, Phys. Zeit, 10, p. 955, 1909; Bull. Bur. Standards, 6, p. 321, 1909. 



