PHYSICAL NATURE OF ANIMAL LIGHT 51 



visible light, A = 0.640/t to A ^ 0.468/*, and assumed that no 

 invisible heat rays were produced. All of the energy of 

 Pyrophorus light would therefore lie in the visible region 

 and its efficiency (light rays -^- heat + light + actinic 

 rays) would be 100 per cent. Later, Langley (1902) rein- 

 vestigated the radiation of Pyrophorus and could detect 

 no heating whatever with the bolometer. * ' A portion of the 

 flame of a standard sperm candle, equal in area to the 

 bright part of the insects, gave under the same circum- 

 stances, a bolometric effect of such magnitude that had 

 the heat of the insect been 1/80,000 as great as that from 

 the candle, it would certainly have been recognized." 

 Coblentz (1912) also, using a vacuum thermopile of Pt 

 and Bi, was unable to detect any infra-red radiation from 

 Photinus pyralis, but found that the temperature of this 

 firefly is slightly lower than the air. These temperature 

 measurements will be discussed in a later chapter. 



The assumption of Langley and Very that the small 

 amount of Pyrophorus radiation passing glass is all light 

 has been called into question by Ives (1910), who points 

 out that Langley and Very failed to use a screen which 

 would cut off either the visible rays or the invisible rays 

 between 3/* and 0.7/*. They really left the question open 

 as to whether the effect of Pyrophorus light on their bolo- 

 meter was due to the visible band of rays or to this plus 

 another band in the infra-red. "The firefly's actual effi- 

 ciency as a light source is dependent to a large degree on 

 the radiation being confined to the visible region. If 

 there should be found infra-red of quantity comparable 

 to the visible, the firefly, while still a very efficient source 

 would not be, as usually supposed, the example of an 

 ideally efficient light produced by nature." 



