CHAPTER XXXI. 



THE EYE AS AN OPTICAL INSTKUMENT. 



The Essential Structure of an Eye. Every visual organ 

 consists primarily of a nervous expansion, provided with 

 end organs by means of which light is enabled to excite 

 nervous impulses, and exposed to the access of objective 

 light; such an expansion is called a retina. By itself, 

 however, a retina would give no visual sensations referable 

 to distinctly limited external objects; it would enable its 

 possessor to tell light from darkness, more light from less 

 light, and (at least in its highly developed forms) light of 

 one color from light of another color; but that would be all. 

 Were our eyes merely retinas we could only tell a printed 

 page from a blank one by the fact that, being partly covered 

 with black letters, (which reflect less light,) it would excite 

 our visual organ less powerfully than the spotless white 

 page would. In order that distinct objects and not merely 

 degrees of luminosity may be seen, some arrangement is 

 needed which shall bring all light entering the eye from 

 one point of a luminous surface to a focus again on one 

 point of the sensitive surface. If A and B (Fig. 121) be 

 two red spots on a black surface, K, and rr be a retina, 

 then rays of light diverging from A would fall equally on 

 all parts of the retina and excite it all a little; so with rays 

 starting from B. The sensation aroused, supposing the 

 retina in connection with the rest of the nervous visual 

 apparatus, would be one of a certain amount of red light 

 reaching the eye; the red spots, as definite objects, would 

 be indistinguishable. If, however, a convex glass lens 

 L (Fig. 122) be put in front of the retina, it will cause to 

 converge again to a single point all the rays from A falling 



