26 A PHYSICAL, STUDY OF THE FIREFLY. 



VIII. LUMINOUS EFFICIENCY AND CANDLE-POWER 



MEASUREMENTS. 



In the experiments on the Cuban firefly, Pyrophorus noctilucus, Langley* 

 and his assistants found that to the eye the insect gave one-eighth as much 

 Hght as an equal area of the candle flame, and that the actual candle-power 

 of the insect was 1/1600 candle. The light from this insect appears to be 

 emitted continuously, but to fluctuate somewhat in intensity. 



Accurate measurements on a fluctuating source, such as the flashes emitted 

 by the local species of fireflies, is, of course, impossible; but it is highly 

 desirable to have some idea of the magnitude of the light emission. A 

 simple "grease spot" photometer w^as therefore provided, the comparison 

 source being a standard sperm candle. 



In the measurements on the glow of the firefly, the photometer screen was 

 placed at 5 meters from the comparison source and the insect, with the 

 luminous segments turned toward the screen, was moved back and forth 

 until the neutral point was reached. This distance varied from i.o, 1.5, 

 to 2.2 cm., depending upon the intensity of the glow and the size of the 

 insect. Some specimens for a short time glowed very strongly, 1/50,000 

 candle, then suddenly decreased to 1/150,000 candle or less. The weakest 

 measurements were 1/250,000 candle; but these were strong as compared 

 with the faint glow observed while photographing the spectrum of the 

 Photinus scintillans. 



Measurements were made of 8 to 10 different specimens, the average 

 intensity of the glow being of the order of 1/50,000 candle. Because of the 

 inability to cause the insect to flash rapidly for any length of time, the 

 measurements were more difficult, and hence uncertain. In this case the 

 insect was held over a glass plate at a fixed distance (about 7.5 cm. from the 

 photometer screen), and the latter was moved back and forth before the 

 comparison source. The main difficulty encountered was the variation in 

 intensity of the flash. A fresh specimen sometimes emitted several strong 

 flashes which would suddenly become weak. 



As in the measurements on the glow, the specimens examined were of two 

 kinds, viz, fireflies that had been in captivity for over a week and fireflies 

 that had just been caught in the fields. Several other simple tests were 

 tried, but no satisfactory measurements were made of the flash, due princi- 

 pally to the great fluctuations in intensity. The measurements on the 

 candle-power of the flash indicated a variation from 1/50 candle to 1/400 

 candle, the predominating values being 1/400 candle. In all, about a dozen 

 measurements of the intenstity of the flash were made on three healthy 

 specimens of Photinus pyralis. It would have been interesting to make 

 similar measurements of the Photuris, but it was too late in the season. As 

 mentioned elsewhere, male specimens of the latter are frequently obtained 

 which emit a "twinkling" flash 3 times per second, and will sometimes con- 

 tinue to do so for I to i^ hours without serious interruption. 



In the previous investigation the radiant efficiency f of the PhoUnus pyralis 

 was found by multiplying the spectral energy curve by factors representing 



*AnnaIs Astrophys. Obs., 2, p. 5, 1902. 



fives and Coblentz, Bull. Bur. Standards, 6, p. 330, 1909. 



