VISION. 



451 



of the whole organ; (t,t), being the tubes dis- 

 posed in radii every where perpendicular to the 

 convex hemispherical 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 them- 

 selves, the same relative situation as the external 

 objects, from which they originally proceeded, 

 and which they will accordingly faithfully re- 

 present. 



The second method, which is nearly the in- 

 verse of the first, consists in admitting the rays 

 through a small aperture into a cavity, on the 

 opposite and internal side of which the retina is 

 expanded, forming a concave, instead of a convex 



40/ 



hemispherical surface. The mode in which this 

 arrangement is calculated to answer the intended 

 purpose will be easily understood by conceiv- 

 ing a chamber (as represented in Fig. 407), into 



