50 THE NATURE OP ANIMAL LIGHT 



radiated in comparison with that of the candle is very 

 small indeed. A more careful study has been made by 

 Langley and Very (1890) with the bolometer. They point 

 out first of all that the total radiation from the most 

 powerful luminous organ (the abdominal one) of Pyro- 

 phorus which affected their bolometer slightly, would, in 

 the same time (10 seconds), be sufficient to raise the tem- 

 perature of an ordinary mercurial thermometer having a 

 bulb 1 cm. in diameter by rather less than 2.3 X 10"^° C. 

 We may thus gain some idea of the magnitude of the meas- 

 urements to be made. The radiation from Pyrophorus 

 which affected their bolometer was shown to be due merely 

 to the "body heat" * of the insect, and it is largely cut 

 off by a plate of glass which is opaque to all wave-lengths 

 of 3ju. or more. These waves are given off by bodies at 

 temperatures below 50° C. and belong "to quite another 

 spectral region to that in which the invisible heat associ- 

 ated with light mainly appears. ' ' Langley and Very then 

 compared the radiatipn from a non-luminous bunsen flame 

 and the Pyrophorus light, interposing a plate of glass in 

 each case, to cut off the waves longer than 3/t, and found 

 several hundred times more radiation in the case of the 

 bunsen burner but, nevertheless, perceptible radiation 

 from Pyrophorus. The former consisted of radiant heaf 

 shorter than A = 3/t and extending up to the visible light 

 rays (A = 0.7/a, since the bunsen flame emitted no light) . 

 The very slight effect of the Pyrophorus radiation must 

 be due to wave-lengths between A. = 3ju and A = 0.468/1, the 

 limit of the Pyrophorus spectrum in the blue. Langley 

 and Very assumed it to be due entirely to the band of 



* Langley and Very evidently supposed that the body temperature of the 

 firefly, like the mammal or bird, is higher than its surroundings. 



