THE SENSE OF VISION. 



747 



dioptric system in its simplest form consists of two adjacent media which have 

 different indices of refraction and whose surface of separation is the segment 

 of a sphere. A line joining the middle of the segment with the centre of the 

 sphere and prolonged in either direction is called the axis of the system. Let 

 the line AP B in Figure 213 represent in section such a spherical surface the 



B 

 FIG. 213. Diagram of simple optical system (after Foster). 



centre of which is at N t the rarer medium being to the left and the denser me- 

 dium to the right of the line. Any ray of light which, in passing from the 

 rarer to the denser medium, is normal to the spherical surface will be unchanged 

 in its direction i. e. will undergo no refraction. Such rays are represented by 

 the lines P, MD, and M f E. If a pencil of rays having its origin in the rarer 

 medium at any point in the axis falls upon the spherical surface, there will be 

 one ray viz. the one which coincides with the axis of the system, which will 

 pass into the second medium unchanged in its direction. This ray is called 

 the principal ray (OP), and its point of intersection (P) with the spherical 

 surface is called the principal point. The centre of the sphere (N) through 

 which the principal ray necessarily passes is called the nodal point. All the 

 other rays in the pencil are refracted toward the principal ray by an amount 



FIG. 214. Diagram to show method of finding principal foci (Neumann). 



which depends, for a given radius of curvature, upon the difference in the 

 refractive power of the media, or, in other words, upon the retardation of light 

 in passing from one medium to the other. If the incident rays have their 

 origin at a point infinitely distant on the axis i. e. if they are parallel to each 

 other they will all be refracted to a point behind the spherical surface known 



