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 towards the external world, while the 

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

 onl}^ light which can reach the latter, under these circum- 

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



Fig. 29.— Diagram showing- the course of rays of liglit from three 

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

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

 with the visual rods. 



direction of a straight line represented by the produced 

 axis of the tubes. 



Su{)pose A — 1 to be nine such tubes, a — i the corre- 

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

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



