54 THE NATURE OF ANIMAL LIGHT 



greater action on the plate than visible light, we must 

 conclude that ultra-violet is absent. Ives (1910) investi- 

 gated the spectrum of Phot inns py rails, using a quartz 

 spectroscope, and found no evidence of ultra-violet radia- 

 tion, at least as far as A = 0.216/u.. 



It will thus be seen that the radiation from the firefly 

 has been very carefully studied and that no waves are 

 given off from A =: 1.5/^ to A = 0.216/* with the exception of 

 the short band (A == 0.67iu- to A r:r= 0.51,a) in the visible, and 

 it is highly probable that no radiation is given off with 

 wave-lengths longer than A = 1.5/a The firefly light 

 remains, then, 100 per cent, efficient, differing from all 

 our artificial sources of light, the best of which does not 

 approach this value. As Langley and Very express it in 

 the title to their paper, it is ^Hhe cheapest form of light,'* 

 not cheapest in the sense of that we can reproduce it 

 commercially at less cost than other lights, but cheaper 

 in the sense that it is the most economical in the energy 

 radiated. This energy is all light and no heat. ^*Cold 

 light'' has actually been developed by the firefly and 

 concerning which *^we know of nothing to prevent our 

 successfully imitating. ' ' 



I have already pointed out that we may also consider 

 the efficiency of a light in relation to the sensibility of 

 our own eye. That is, we take into account not only the 

 energy distribution in the spectrum of the light but also 

 the fact that different wave-lengths of an equal energy 

 spectrum affect our eye very differently. As the normal 

 light-adapted eye is most sensitive to yellow green of 

 A = 0.565/A, monochromatic light of this wave-length will 

 appear much brighter than monochromatic light of any 

 other wave-length with the same energy. Monochromatic 



