vi ])IOPTE1C MECHANISM OF THE EYE 279 



pagation of light in the two media. If V and V indicate the 



y 

 velocity in the two media, then n = -==, 



(b) Refraction of Light in a Simple Convergent System. The 

 laws of refraction are the same when two refractive media are 

 separated by a curved surface and by a plane surface. The differ- 

 ence in the effects obtained depends solely on the fact that the 

 di rection of the normal varies from one point to another when the 

 surface is curved, while it is constant on a plane surface. Let MM ' 

 (Fig. 118) represent two differently refracting media, separated by 

 the spherical surface AB, the centre of curvature of which lies at 

 C in the plane of section ; all radii of this circle drawn from G 

 would fall perpendicularly in the corresponding tangents of the 

 spherical surface, and are called directive lines. Directive lines 

 which pass from M to M' in a plane perpendicular to the corre- 

 sponding tangents, as PC, P'C, are not refractedjiand meet at G, 



FIG. 118. Refraction of a ray that passes through two media separated by a spherical surface. 



which is, as we shall see, the nodal point of the convergent system. 

 All other rays, on the contrary, which fall obliquely upon the 

 surface AB from the point P, are refracted on reaching the 

 medium M'. If we suppose that another ray from point P falls 

 obliquely on the curve AB, it is refracted approximately into the 

 direction of P C, so that at / it meets the line PC, which is called 

 the principal axis or optical axis of the system. All other rays 

 from the point P that fall on the spherical surface are more or less 

 refracted according to their greater or less obliquity, so that they 

 all converge towards /. This, however, is true only of the cone of 

 rays that form an acute angle with the optical axis, i.e. the rays 

 which fall on the refractive surface at an angle closely approximat- 

 ing to a right angle. For such rays point / coincides with the 

 image of the object represented at point P. 



If, on the contrary, the luminous point lies in M', the more 

 highly refractive medium, the rays from / that fall on the con- 

 cavity of the spherical surface converge after refraction at P, for 

 by the law of reciprocity light-rays passing from M' to M take the 



