PHYSICAL NATURE OF ANIMAL LIGHT 57 



pared with that of a carbon glow lamp by measuring the 

 amount of photochemical change produced on panchro- 

 matic photographic plates. Fig. 11 gives the energy 

 curves of various fireflies and the carbon glow lamp in the 

 same spectral region. The visual sensibility curve used 

 by Ives and Coblentz is that of Nutting (1908, 1911), based 

 on Konig^s data. It is reproduced in Fig. 6. The latest 

 visibility curve is that of Hyde, Forsyth and Cady (1918), 



Fig. 13 — Luminous efficiency of the 4-watt carbon glow lamp, shaded area h- total area 



{after Ives and Coblentz) . 



reproduced in Fig. 12. It is based on observations of 

 twenty-nine individuals. As individuals vary consider- 

 ably in their sensibility to different wave-lengths, the 

 visibility curve represents an average, but it is the only 

 standard we have with which to evaluate the energy we 

 call light. Color-blind individuals would have a visibility 

 curve very different from normal individuals. Composite 

 curves showing the luminous efficiency of the 4-watt carbon 

 glow lamp and the firefly, both in relation to visibility, 

 are given in Figs. 13 and 14, respectively. In these figures 



