THE SENSE OF VISION. 749 



are as follows : layer of tears, cornea, aqueous humor, anterior capsule of lens, 

 lens, posterior capsule of lens, vitreous humor. The surfaces are those which 

 separate the successive media from each other and that which separates the tear 

 layer from the air. For purposes of practical calculation the number of sur- 

 faces and media may be reduced to three. In the first place, the layer of tears 

 which moistens the surface of the cornea has the same index of refraction as 

 the aqueous humor. Hence the index of refraction of the cornea may be left 

 out of account, since, having practically parallel surfaces and being bounded 

 on both sides by substances having the same in^hsx of refraction, it does not 

 influence the direction of rays of light passing through it. For this same 

 reason objects seen obliquely through a window appear in their true direction, 

 the refraction of the rays of light on entering the glass being equal in amount 

 and opposite in direction to that which occurs in leaving it. For purposes of 

 optical calculation we may, therefore, disregard the refraction of the cornea 

 (which, moreover, does not differ materially from that of the aqueous humor), 

 and imagine the aqueous humor extending forward to the anterior surface of 

 the layer of tears which bathes the corneal epithelium. Furthermore, the cap- 

 sule of the lens has the same index of refraction as the outer layer of the lens 

 itself, and for optical purposes may be regarded as replaced by it. Hence 

 the optical apparatus of the eye may be regarded as consisting of the fol- 

 lowing three refracting media: Aqueous humor, index of refraction 1.33; 

 lens, average index of refraction 1.45; vitreous humor, index of refraction 

 1.33. The surfaces at which refraction occurs are also three in number : An- 

 terior surface of cornea, radius of curvature 8 millimeters ; anterior surface 

 of lens, radius of curvature 10 millimeters ; posterior surface of lens, radius of 

 curvature 6 millimeters. It will thus be seen that the anterior surface of the 

 lens is less and the posterior surface more convex than the cornea. 



To the values of the optical constants of the eye as above given may be 

 added the following : Distance from the anterior surface of the cornea to the 

 anterior surface of the lens, 3.6 millimeters ; distance from the posterior sur- 

 face of the lens to the retina, 15. millimeters ; thickness of lens, 3.6 millimeters. 



The methods usually employed for determining these constants are the fol- 

 lowing : The indices of refraction of the aqueous and vitreous humor are 

 determined by filling the space between a glass lens and a glass plate with the 

 fresh humor. The aqueous or vitreous humor thus forms a convex or concave 

 lens, from the form and focal distance of which the index can be calculated. 

 Another method consists in placing a thin layer of the medium between the 

 hypothenuse surfaces of two right-angled prisms and determining the angle at 

 which total internal reflection takes place. In the case of the crystalline lens 

 the index is found by determining its focal distance as for an ordinary lens, 

 and solving the equation which expresses the value of the index in terms of 

 radius of curvature and focal distance, thickness, and focal length. The 

 refractive index of the lens increases from the surface toward the centre, a 

 peculiarity which tends to correct the disturbances due to spherical aberration, 

 as well as to increase the refractive power of the lens as a whole. 



