THREE POSSIBLE MODES OF SIGHT. 



119 



screen. In this case the rays from the point c can 

 pass straight through the orifice o, and fall on the 

 retina ot an eye, or on a flat surface at c'. There is 

 no other direction in which the rays from c could pass 

 through 0. In the same way, 

 the light from a would fall on 

 the point a\ that from h on h\ 

 from d on d\ and e on e'. 



The results which would be 

 given in this way would be, 

 however, very imperfect, and, 

 as a matter of fact, no eye con- 

 structed on this system is 

 known to exist. 



Secondly, let a number of 

 transparent tubes or cones with opaque walls be ranged 

 side by side in front of the retina, and separated from 

 one another by black pigment. In this case the only 

 light which can reach the optic nerve will be that which 

 falls on any given tube in the direction of its axis. 



Fig. 75. 



For instance, in Fig. 75 the light from a will pass to a\ 

 that from h to h', that from c to c\ and so on. The 

 light from c, which falls on the other tubes, will not 



