58 



THE NATURE OF ANIMAL LIGHT 



the luminous efficiency is the shaded area -4- total area, 

 0.43 per cent, for the carbon glow lamp and 99.5 per cent, 

 for the firefly, ^^ these numbers representing the relative 

 amounts of light (measured on a photometer) for equal 

 amounts of radiated energy — a striking illustration of the 



Fig. 14. — Luminous efficiency of the firefly, shaded area -r total area {after Ives and 



Coblentz) . 



wastefulness of artificial methods of light production. 

 From the specific consumption of the tungsten lamp (1.6 

 watts per spherical candle) and the mercury arc (.55 watts 

 per spherical candle) we obtained by comparison with the 

 carbon filament that their luminous efficiencies are 1.3 and 

 3.8 per cent. The most efficient artificial illuminant there- 

 fore has about 4 per cent, of the luminous efficiency of the 



