450 



THE SENSORIAL FUNCTIONS. 



I 



rays of light, while they are transmitted through 

 the same medium, proceed in straight lines, the 

 simplest mode of accomplishing the proposed end 

 would be to admit into the eye, and convey to 

 each particular point of the retina, only a single 

 ray proceeding directly from that part of the ob- 

 ject which is to be depicted on it, and to exclude 

 all other rays. For carrying this design into 

 effect we have the choice of two methods, both 

 of which we find resorted to by nature under 

 different circumstances. 



The first method consists in providing for 

 each of these single rays a separate tube, with 

 darkened sides, allowing the ray which traverses 

 it, and no other, to fall on its respective point of 

 the retina, which is to be applied at the opposite 

 end of the tube. The most convenient form to 

 be given to the surface of the retina, which is to 



be spread out to receive the 

 rays from all these tubes, 

 appears to be that of a con- 

 vex hemisphere ; and the 

 most eligible distribution of 

 the tubes is the placing them 

 so as to constitute diverging 

 radii, perpendicular, in 

 every part, to the surface of 

 the retina. This arrange- 

 ment will be understood by 

 reference to Fig. 406, which represents a section 



