306 THE SPECIAL SENSES. 



The three points at which the hght is refracted are indicated 

 in the accompanying schema (Fig. 126). The refractive surfaces 

 of the eye may be considered as being composed of a concavo-convex 

 lens, the cornea and aqueous humor, and a biconvex lens, the 

 crystalline Jens. In a system of this kind, composed of several 

 refractive media, it has been shown that to construct geometrically 



the path of the rays it is 

 necessary to know six 

 points ; these are the six car- 

 dinal points or optical con- 

 stants of Gauss, — namely, 

 the anterior and the poste- 

 rior focal distance, the two 

 nodal points, and the two 

 principal points. So far 



Fig. 126.— Diagram tr illu.strate the surfaces aS the eye is COUCemed, it 

 in the eye at which the rays of light are chiefly , , , , i , i 



refracted. has been shown that the 



path of the rays of light 

 may be represented with sufficient accuracy by employing what is 

 known as the reduced schematic eye of Listing, in which the 

 refraction is supposed to take place at a single convex surface 

 separating two media, the air on one side and the humor of the eye 

 on the other, the latter having a refractive index of 1.33 (see Fig. 

 127). In this reduced eye the position of the ideal refracting 

 surface c' lies in the aqueous humor, at a distance of 2.1 mms. from 

 the anterior surface of the 

 cornea, and the position of 

 the nodal point or optical 

 center — that is, the center 

 of curvature of the ideal 

 refracting surface, lies in 

 the crystalline lens at n, a 

 distance of 7.3 mms. from 

 the anterior surface of the p. ,.„ y.. * n * * *u ^ i 



Fig. 127. — Diagram to illustrate the reduced 



cornea. The principal fo- or schematic eye with a single refracting surface 



... , . . . separating two media of different densities: c', 



Cal distance lOl this reiract- the ideal refracting surface situated 2.1 mms. 



J. ,. , 1. • behind the anterior surface of real cornea: n, 



ing SUriace lies at a dlS- the nodal point, or center of curvature of the 



+ ori«« r^f on 7 Tvirvno titKt^I^ surface c', and 15.5 mms. in front of retina, 



tance OI ZU.i mms., Wnicn The eyeball is supposed to be filled with a uni- 



wnnlrl ho pniiivqlpnf fr> form substance having a refractive index of 1.33, 



WOUlQ De equivalent, to g^^^, ^^ ^^^^ ^^ ^j^g vitreous humor. 



22.8 mms. (20.7 + 2.1) 



from the actual surface of the cornea and 15.5 mms. (22.8 — 7.3) 

 from the nodal point. In the eye at rest this principal focal 

 distance coincides with the retina, since the refracting surfaces 

 in the normal resting eye are so formed that parallel rays (rays 

 from distant objects) are brought to a focus on the retina. To 

 show the formation of the image of an external object on the retina 



