PHYSICAL NATURE OF ANIMAL LIGHT 41 



The shift in sensibility of the eye occurs in illumina- 

 tions of between 0.5 and 50 metre-candles and represents a 

 change from central cone vision (high intensities) to 

 peripheral rod vision (low intensities). The fovea cen- 

 tralis lacks rods and this part of the eye becomes prac- 

 tically color blind at very low intensities of light. Below 

 0.5 and above 50-metre candles visibility varies but little 



WAVE LENSTH 



Fig. 6. — Visibility curves for three illuminations showing the shift in region of maximum 

 visibility, or Purkinje phenomenon {.after Nutting). 



with change in intensity. It is, clearly necessary then to 

 distinguish between the physical objective phenomenon 

 of light and the physiological subjective sensation of light. 

 It is a fact that different luminous animals produce 

 light of quite different colors as judged by our eye. A 

 range of spectral tints has been described which extends 

 from red to violet but "yellowish," "greenish" and 

 ' ' bluish ' ' tints are commonest. Indeed one or two animals 

 possess several luminous organs emitting lights of differ- 

 ent colors. This is true in a South American firefly, Phen- 



