VISION. 411 



parallel, which in mathematical strictness they 

 never are. The focus of the rays refracted by a 

 convex lens is more remote in proportion as the 

 rays are more divergent; or, in other words, pro- 

 ceed from nearer objects. This is illustrated by 

 Figures 412, 413, and 414 ; to which I shall again 

 have occasion to refer in the sequel. 



^ 3. Structure of the Eye. 



One of the many points of superiority which the 

 eye possesses over the ordinary camera obscura is 

 derived from its spherical shape, adapting the retina 

 to receive every portion of the images produced by 

 refraction, which are themselves curved ; whereas 

 had they been received on a plane surface, as they 

 usually are in a camera obscura, a considerable 

 portion of the image would have been indistinct. 

 This spherical form is preserved by means of the 

 firm membranes which protect the eye, and which 

 are termed its Coats; the transparent media which 

 they enclose, and which effect the convergence 

 of the rays, are termed the Humours of the Eye. 

 There are in this organ three principal coats, and 

 three humours ; composing altogether what is called 

 the Globe of the Eye. Fig. 415, which gives an 

 enlarged view of a horizontal section of the right 

 eye, exhibits distinctly all these parts. 



The outermost coat (s), which is termed the 

 Sclerotica, is exceedingly firm and dense, and gives 

 to the globe of the eye the mechanical support it 

 requires for the performance of its delicate func- 

 tions. It is perforated behind by the optic nerve 



