286 THE SPECIAL SENSES. 



in the accompanying schema (Fig. 119). 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 lens. 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. 119. Diagram to illustrate the surfaces as the eye is Concerned, it 

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



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 media of the eye 

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

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

 surface 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, c', lies in 

 the crystalline lens at n, a 

 distance of 7.3 mms. from 

 the anterior surface of the 



. Fig. 120. Diagram to illustrate the reduced 



COmea. The principal focal or schematic eye with a single refracting surface 



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



distance IOr thlS retracting the ideal refracting surface situated 2.1 mms. 



e ,. ... . f. behind the anterior surface of real cornea: n, 



Surface lies at a distance 01 the nodal point, or center of curvature of the 



OO Q mmc ll^r^ +V. on+^ surface c', and 15.5 mms. in front of retina. 



ll.?S mms. Denma tUe ante- The eyeball is supposed to be filled with a uni- 



r\nv ciirfapp nf tViA onmvn rtr form substance having a refractive index of 1.33, 



noi sunace or tne cornea or equal to that of the Yitieoua humor . 

 (22.8 7.3) at a distance of 



15.5 mms. behind 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 

 it suffices, therefore, to use a construction such as is represented in 



