44 THE NATURE OF ANIMAL LIGHT 



A = 0.640/-1. It consists, then, of a broad band chiefly in the 

 green and yellow. But, ^Svould the light not extend 

 farther were it bright enough to be seen! ... if the light 

 of the insect were as bright as that of the sun would it not 

 extend equally far on either side of the spectrum f'^ ''It 

 is impossible to increase the intrinsic brilliancy by any 

 optical device, but if it be impossible to make the light 

 of the insect as bright as that of the sun, it is on the other 

 hand quite possible to make the light of the sun no 

 brighter than that of the insect . . .'' Langley and Very 

 investigated this question, forming a solar spectrum from 

 sunlight of the same intensity as that of Pyrophorus and a 

 Pyrophorus spectrum together in the same field of the 

 spectroscope. The latter was very much shorter than the 

 solar spectrum, showing that its length was not due to 

 weakness of the red and blue rays but to their absence. 

 Later Ives and Coblentz (1910) photographed the spec- 

 trum of a firefly {Photinus py rails), together with that of 

 a carbon glow lamp, on plates sensitive to all wave-lengths 

 of visible rays under conditions which would have re- 

 corded all visible radiations given off. They found the 

 spectrum to extend only from A =^ 0.51ft to A = 0.67/^ (Fig. 

 7). Another species of firefly {Plioturis pennsylvanica) 

 was found by Coblentz (1912) to give a spectrum extend- 

 ing from A^O.Slju. to A =:= 0.59ft (Fig. 8). The Photinus 

 light extends much further into the red and it is easy to 

 distinguish between Photinus and Photiiris in nature, 

 merely by the reddish tint of the light of the former. 

 These photographic records show conclusively that the 

 color of the light of luminous animals is not a subjective 

 phenomenon due to the Purkinje effect and the low in- 

 tensity of the light, but is real, an actual difference in spec- 



