7 o2 TEXT-BOOK OF PHYSIOLOGY 



tically parallel, and as the index of refraction of the aqueous humor is the 

 same as that of the cornea, they may be regarded as but one medium. The 

 refracting surfaces may, therefore, be reduced to the anterior surface of the 

 cornea, the anterior surface of the lens, and the posterior surface of the 

 lens. 1 



'Parallel rays of light entering the eye pass from air, with an index of re- 

 fraction of 1.00025, into the cornea, with an index of refraction of 1.3365. 

 In passing from the rarer into the denser medium they undergo refraction 

 in accordance with the laws of optics and are rendered somewhat convergent. 



The extent of this first refraction 

 and convergence is sufficiently 

 great to bring parallel rays, if con- 

 tinued, to a focus about 10 mm. 

 behind the retina. This would be 

 the condition in aphakia whether 

 the lens is congenitally absent or 

 has been removed by surgical pro- 

 cedures. Perfect vision, however, 

 FIG. 288. REFRACTION OF HOMOCENTRIC . ,, 



RAYS AND THE FORMATION OF AN IMAGE. requires that the convergence of 



the light must be great enough to 



bring the focal point, the image, on the retina. This is accomplished by 

 the introduction of an additional refracting body, the lens. On entering 

 the lens the rays are for the same reason i.e., the passage from a rarer into 

 a denser medium again refracted and converged, and if continued would 

 come to a focus about 6.5 mm. behind the retina. On passing from the lens 

 into the vitreous i.e., from a denser into a rarer medium the rays are 

 once more converged and to an extent sufficient to focalize them on the 

 retina (Fig. 288). 



While it is thus possible to follow the rays geometrically through these 

 media by means of the above-mentioned factors, the procedure is attended 

 with many difficulties. Moreover, as the relations all change when rays 

 enter the eye from objects situated progressively nearer the eye, a separate 

 calculation is necessitated for each distance for the determination of the 

 size of the image. 



A method by which these difficulties are much reduced was suggested 

 by Gauss and developed by Listing. It was demonstrated by Gauss that 

 in every complicated system of refracting media separated by centered 

 spheric surfaces there may be assumed certain ideal or cardinal points, to 

 which the system may be reduced, and which, if their relative position and 

 properties be known, permit of the determination, either by calculation 

 or geometric construction, of the path of the refracted ray, and the position 

 and size of the image in the last medium, if those of the object in the first 

 medium be known. 



Every dioptric system can be replaced, as Gauss showed, by a single 

 system composed of six cardinal points and six planes perpendicular to the 



1 Strictly speaking, the posterior surface of the cornea is not parallel to the anterior surface, 

 and the index of refraction of the cornea is a trifle greater than that of the aqueous humor, viz.: 

 1.377. But as the increase in the corneal refraction due to the higher index is almost exactly 

 counteracted by a decrease in refraction due to the higher curvature of the posterior corneal 

 surface, the usual assumptions furnish quite accurate results. 



