CHAPTER XXXII. 



THE EYE AS AN OPTICAL INSTRUMENT. 



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 an- 

 other 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 power- 

 fully than the spotless white page would. In order that dis- 

 tinct 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. 139) 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, suppos- 

 ing 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. 140) be put in front of the retina, it will cause to 

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

 upon it; so, too, with the rays from B : and if the focal dis- 

 tance of the lens be properly adjusted these points of conver- 

 gence will both lie on the retina, that for rays from A at , 

 and that for rays from B at b. The sensitive surface would 

 then only be excited at two limited and separated points by 



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