VISION. 



403 



proceeding directly from that part of the object 

 which is to be depicted on it, aiifl "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 convex 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 arrangement will be 

 understood by reference to Fig. 406, 

 which represents a section of the 

 whole organ ; t, t, being the tubes 

 disposed in radii every where per- 

 pendicular to the convex hemis- 

 pherical surface of the retina (r). 

 Thus will an image be formed, 

 composed of the direct rays from 

 each respective point of the objects, to which the 

 tubes are directed ; and these points of the image 

 will have, among themselves, the same relative 

 situation as the external objects, from which they 

 originally proceeded, and which they will accord- 

 ingly faithfully represent. 



The second method, which is nearly the inverse 



