VISION. 211 



i.e., though curved surfaces they are parallel and form a 

 case under the following theorem: "If a ray pass from 

 any medium through a denser medium which is bounded 

 by two parallel planes it emerges from the denser medium 

 in a line parallel to its course before entering that 

 medium." It is customary at this point to take the ante- 

 rior surface of the cornea as the first refractive surface 

 and IL 1.3365. 



Notice that the index of refraction of the aqueous humor 

 and vitreous humor are the same. It is now evident that 

 we have to deal with three media [air, aqueous or vitreous 

 humor, and lens], with three surfaces [ant. corneal surface, 

 ant. and post, lens surface], whose radii are 7.829, 6 and 

 10 respectively. But even this great step toward simpli- 

 fying the problem leaves us with a long road before us un- 

 less we can find a short cut. " It has been shown mathe- 

 matically that a complex optical system consisting of sev- 

 eral surfaces and media, centered on a common optical axis, 

 may be treated as if it consisted of two surfaces only." 

 [Text-book of Physiology Foster, 1891 vol. IV., pg. 9.] 

 The location of these surfaces and the cardinal points are 

 given as follows by Landolt : 



A. The normal eye. 



The point r (Fig. 30.) where the principal axis cuts the 

 cornea is 22.8237 mm. from the second principal focus f 

 (the retina) ; c, the center of curvature of the cornea; s, the 

 point where the optical axis cuts the anterior surface of 

 the lens, is 3.6 mm. from r, the point where the optical 

 axis cuts the posterior surface of the lens 7.2 mm. from 

 r; 1, the center of curvature of ant. surface of lens; 1', 

 the center of curvature of posterior surface of lens. 



B. The accurate mathematical reduction. 



The reduction referred to in the text above is represented 

 by the two refractive surfaces with nodal points n and n' 



