CHAPTER XV 

 THE EYE AS AN OPTICAL INSTRUMENT 



The Essential Structure of an Eye. Every visual organ con- 

 sists 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. 78) be two red spots on a black surface, K, and rr be a ret- 

 ina, 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 connec- 

 tion 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. 79) be put in front of the retina, it will 

 cause to converge again to a single point all the rays from A fall- 

 ing upon it; so, too, with the rays from B: and if the focal distance 

 of the lens be properly adjusted these points of convergence will 

 both lie on the retina, that for rays from A at a, and that for rays 

 from B at b. The sensitive surface would then only be excited at 

 two limited and separated points by the red light emanating from 

 the spots; consequently only some of its end organs and nerve- 



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