440 THE OUTGO OF ENERGY 



spherical surface 1 (the cornea) and a double con- 

 vex lens (the crystalline lens) it will be advis- 

 able to consider first the cardinal points of the 

 cornea (System A), next those of the lens (Sys- 

 tem B), and finally those of the two combined 

 as in the eye (System C). 



CARDINAL POINTS OF THE CORNEA (SYSTEM A) 



Construction Drawing of System A. Draw 



a horizontal line to serve as the optical axis. 2 

 Take any point, k, in this line for a centre of 

 curvature. 3 From this point describe an arc 



1 The cornea is not strictly a spherical surface, but more 

 nearly that produced by the revolution of an ellipse about its 

 major axis. 



2 This construction drawing should be placed near the top of 

 the page, in order to permit the construction drawings for the 

 lens and the compound optical system to be made beneath it. 

 All these drawings will be the natural size. 



8 The following list will be found convenient : 

 k t centre of curvature. 

 r t radius of curvature. 



hi, intersection of the first spherical surface of any system with 

 its principal axis (" first " is used in the sense of nearest 

 the source of light). 

 hz, intersection of the second spherical surface with its principal 



axis. 



n lt the first medium, that which bounds the refracting surface 

 on the side from which the ray comes ; also the refractive 

 { index of this medium. 



ttj, The second medium, that which bounds the refracting sur- 

 face on the side from which the ray emerges ; also the 

 refractive index of this medium. 



