THE THEORY OF MOSAIC VISION. 



123 



Each visual pyramid, isolated from its fellows by its coat 

 of pigment, may be supposed, in fact, to play the part of a 

 very narrow straight tube, with blackened walls, one end 

 of which is turned tow^ards the external world, while the 

 other incloses the extremity of one of the nerve fibres. The 

 only light which can reach the latter, under these circum- 

 stances, is such as proceeds from points which lie in the 



2*-- 



FiG. 29. — Diagram showing the course of rays of light from three 

 points X, y, z, through the nine visual rods (supposed to be empty- 

 tubes) A — I of a compound eye ; a—i, the nerve fibres connected 

 with the visual rods. 



direction of a straight line represented b}' the produced 

 axis of the tubes. 



Suppose A — I to be nine such tubes, a — i the corre- 

 sponding nerve fibres, and x y z three points from which 

 hght proceeds. Then it will be obvious that the only light 



