840 FORMATION OF RETINAL IMAGES. [BOOK in. 



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

 are dealing. This line is called the optic axis. 



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

 tive to the second more refractive medium, cut the surface nor- 

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

 others, such as md, m'e, 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, 

 Fy 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 v the rays of light passing from which, 



FIG. 140. DIAGRAM OF SIMPLE OPTICAL SYSTEM. 



such as F^, 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 

 by several surfaces, the latter differing from each other in curva- 

 ture, though being approximately spherical. Rays of light in 

 passing from an external object to the retina traverse in succes- 

 sion the following surfaces and media : the anterior surface of 

 the cornea, the substance of the cornea, the posterior surface 

 of the cornea, the aqueous humour, the anterior surface of the 

 lens, the substance of the lens, the posterior surface of the lens, 



