VISION. 



207 



The center of curvature of the spherical surface 

 (n) in the nodal point. 



That radius which is the center of symmetry of 

 the dioptric system (e. g., n p.) is called the princi- 

 pal axis of the system. In this axis lie the first and 

 second principal foci, f and f respectively. The point 

 where the optical axis cuts the spherical surface 

 (p) is called the principal point. The plane tangent 

 to the spherical surface at this point is the principal 



FIG. 29. 



FIG. 29. A. Showing the cardinal points of a simple dioptric sys- 

 tem, n, nodal point; R p n, principal axis; p, principal point; f, f , 

 principal foci. 



FIG. 29. B. Showing the relation of the visual angle, v and the 

 size of object and image to values p and n. 



plane. Planes perpendicular to the optical axis at 

 f and f are called the first and second principal focal 

 planes respectively. 



Problem. Given the radius of curvature and the 

 index of refraction to locate upon the principal axis 

 the principal foci. 



Neumann has given the following construction: 



