CHAP, in.] SIGHT. 1145 



the curvature of the surface, (2) the relative refractive powers of 

 the media. 



Such a simple optical system is represented in Fig. 135, where 

 apb represents, in section, a curved (spherical) surface separat- 

 ing a less refractive medium, on the left hand towards 0, from a 

 more refractive medium on the right hand towards A. The 

 surface in question is symmetrically placed as regards the 

 line OA, which falling normal (perpendicular) to the surface at 

 p passes through the centre n of the sphere with whose surface 

 we are dealing. This line is called the optic axis. 



FIG. 135. DIAGRAM OF SIMPLE OPTICAL SYSTEM. 



All rays of light which, in passing from the first less refractive 

 to the second more refractive medium, cut the surface normally, 

 such as the one, Op, in the line of the optic axis, and others, such 

 as md, me, undergo no refraction ; all such rays are continued on 

 as straight lines, and all pass through n the centre of the sphere 

 or nodal point. All other rays passing from the first to the 

 second medium are- refracted. Of these all those which lie in 

 the first medium parallel to the optic axis, such as cd, are so 

 refracted as to meet in the second medium at a point, F 2 , on 

 the optic axis ; this is called the principal posterior (or second) 

 focus. On the optic axis in the first medium there is another 

 important point, F 1} the rays of light passing from which, such as 

 F^e, are so refracted in passing into the second medium as to 

 become parallel, ef, to the optic axis ; this point is called the 

 principal anterior (or first) focus. The point at which the optic 

 axis cuts the surface is, for reasons which we shall see presently, 

 called the principal point. The above points, viz. the posterior 

 and the anterior principal foci, the nodal point, and the principal 

 point are the cardinal points of such an optical system. 



Such a simple system, however, does not represent the optical 

 conditions of the eye, for this consists of several media bounded 



