846 THE SENSE OF SIGHT 



CHAPTER LXXII 

 THE FORMATION OF THE IMAGE UPON THE RETINA 



The Reduced or Schematic Eye. — The eye consists of two parts, 

 namely, the hemispherically expanded retina with its mosaically ar- 

 ranged sensory elements and a number of adjuncts which form a 

 dioptric mechanism for projecting the light rays upon this receptor. 

 Having previously studied the structure and function of these parts 

 separately, we are now in a more favorable position to deal with them 

 collectively and to see how they are capable of forming a real image of 

 external objects in their correct spatial relationships upon the retinal 

 surface. This analysis should not be attended by undue difficulties, 

 because it is based essentially upon the data pertaining to the refrac- 

 tion by biconvex lenses given in one of the preceding chapters. 



The normal or emmetropic eye is constructed in such a way that 

 the different rays of light are brought to a precise focus upon the retina. 



This refraction, however, involves 

 not only those rays which pursue a 

 course parallel to its visual axis, but 

 also those which are projected toward 

 it in a divergent direction and 

 would otherwise be lost to it. This 

 power it exerts by virtue of its ability 

 to accommodate for near and far 

 objects. But while the process of 

 Fig. 446.-DIAGKAM to Illustrate refraction in our eye is essentially 



THE Position of the Chief Points of the same as that exhibited by bi- 



Refraction in Our Eye. convex lenses, the fact that several 



A Cornea; B, anterior surface of ^j ^ j^ ^^ j j^ ^ ^ ^ j^ 



lens; C, posterior surface of lens. • ^^m ^ * i- 



matters more difficult. A biconvex 

 lens changes the course of the ray in two places, namely, at the point 

 where the latter enters the denser medium and again where it leaves 

 it. Upon its passing from the rarer into the denser medium it is 

 refracted toward the perpendicular, and upon its passing from the 

 denser into the rarer medium, away from the perpendicular. Our 

 eye contains a large number of these points of refraction, chief 

 among which are the anterior surface of the cornea and aqueous humor, 

 the anterior and posterior surfaces of the lens, together with the an- 

 terior surface of the vitreous humor (Fig. 446) . In fact, the entering 

 ray of light first meets with a layer of tears, the refractive power of 

 which is considerable. Inside the cornea it is not deviated very mate- 

 rially, because the anterior and posterior surfaces of this medium are 



