VISION IN INSECTS. 



139 



the light. If rays of different colours, given out 

 from the points «, 6, c, d, fall upon the eye, the 

 cone h will be illuminated throughout its whole 

 length by the ray d' , which traverses this cone in the 

 direction of its long axis. The other cones situated 

 in the vicinity of the line m d will not be illumi- 

 nated as far as their internal extremity by the rays 

 from d, which penetrate less and less deeply into the 

 neighbouring cones, in proportion as they become 

 more remote from the line m d. The nervous fila- 

 ment m, corresponding to the cone h, is conse- 

 quently impressed with the ray d' ; other rays from 

 d, being absorbed by the pigment investing the 

 neighbouring cones, will of course produce no effect 

 on any nervous filament placed out of the line in d. 

 The coloured ray d' is therefore perceived only by 

 means of the filament m, on which latter alone it 

 impinges. So also the ray c', given out at the point 



