REFRACTION IN THE EYE 441 



with the radius r = 7.829 mm., the radius of 

 curvature of the cornea. The intersection of 

 the arc with the axis is termed the principal 

 point ^ of the axial ray. The spherical surface 

 separates two media : n v the air, and n 2 , the 

 aqueous humor. 



Principal Focal Distances. 1. Kays passing 

 from the first medium through the cornea into 

 the second medium unite nearly in a point, the 

 posterior principal focus, < 2 . The distance, Ai< 2 , 

 between this point and the principal point is the 

 posterior principal focal distance, F 2 . Kays pass- 

 ing from the aqueous humor through the cornea 



lf anterior principal focus. 



# 2 , posterior principal focus. 



FI, anterior focal distance. 



F 2 , posterior focal distance. 



/ l5 anterior conjugate focal distance. 



/ 2 , posterior conjugate focal distance. 



o, optical centre. 

 Ki t first nodal point. 

 J5T 2 , second nodal point. 

 HI, first principal point. 

 JET 2 , second principal point. 



s, point between two refracting surfaces at which an object 

 must be in order that the images of the object formed by 

 the refracting surfaces shall be similar, i. e., images of 

 one another, lying therefore in the principal surfaces. 

 Where confusion might arise in applying these terms to Sys- 

 tem A, B, or C, they will be distinguished by placing with them 

 the letters A, B, C, respectively. Thus the anterior focal dis- 

 tance of the lens will be written FI B, wherever it might other- 

 wise be confused with that of System A or C. 



