442 THE OUTGO OF ENERGY 



to the air, parallel to and near the axis, unite in 

 front of the cornea nearly in a point, the anterior 

 principal focus, fa. The distance, h l fa, between 

 this point and the principal point is the anterior 

 principal focal distance, FI. The principal focal 

 distances are proportional to the coefficients of 

 refraction of the first and last media. The pos- 

 terior principal focal distance is calculated by 

 the formula 1 



( la > F * = ^ 



In comparing refractive powers the air, n lt is 

 taken as the unit. , Thus the formula becomes 



The anterior principal focal distance is calcu- 

 lated by tne formula 



(2 a) J1---3L1L 



n 2 ni 



As ni = unity, the formula becomes 



(2b) *i = ^ '"" 



n z 1 



The refractive index, n it of the air = 1 ; accord- 



1 For the derivation of the formulas in this chapter the 

 reader is referred to the works of Bonders (Accommodation and 

 Refraction of the Eye) and Helmholtz (Handbuch der physio- 

 logischen Optik). 



