460 THE OUTGO OF ENERGY 



<j, the anterior principal focus, the focus of 

 rays parallel in the vitreous. 



</> 2 , the posterior principal focus, the focus 

 of rays parallel in the air. 



h </>! = FI, the anterior focal distance, = 15 mm. 



h <f> z = j^ 2 , the posterior focal distance, = 20 mm. 



n, _ 4 _ ^ _ 20 



w x ~~ 3 ~~ F 1 ~~ 15' 



With the reduced eye many calculations may 

 be rapidly and easily performed. 



