452 THE OUTGO OF ENERGY 



by the cornea will be of equal size with the image 

 of it formed by the crystalline lens. These simi- 

 lar images will lie in the principal surfaces. The 

 image which the cornea forms of the point s will 

 be the first principal point, HI of System C, and 

 the image which the lens forms of s will be the 

 second principal point, H z of System C. The 

 point s lies between li\ of System A and HI of 

 System B, at distances proportional to the pos- 

 terior focal distance (^ T 2 31.095 mm.), of System 

 A and the anterior focal distance (Fi = 50.617 

 mm.) of System B. The distance between hi, which 

 lies at the anterior surface of the cornea, and 

 HiB, which lies 2.126 mm. behind the anterior 

 surface of the lens, is 3.6 mm. (the distance be- 

 tween the cornea and the lens) plus 2.126 mm. 

 = 5.726 mm. This distance is to be divided in 

 the proportion 50.617 : 31.095. 



50.617 + 31.095 : 31.095 :: 5.726 : x. 



#=2.179. Hence s lies 2.179 mm. behind the 

 cornea, and 5.726 2.179 = 3.547 mm. in front of 

 the anterior principal point of the crystalline lens. 

 The first or anterior principal point of the eye, 

 HiC, is the virtual image of s formed by the 

 cornea; it lies at the conjugate focus of s, and 

 its position is determined by the formula (3 b), 

 page 444. 



