REFRACTION IN THE EYE 455 



converged to the point < 2 A, situated 31.095 mm. 

 behind the cornea. On their way they are further 

 refracted by System B. Hi B is 5.726 mm. behind 

 the cornea. The point c/> 2 A is 31.095 5.726 

 = 25.369 mm. behind HI. Calculated from H 2 B, 

 the posterior principal focal distance F 2 of System 

 B is 50.617 mm. The posterior focal distance of 

 System C is calculated by the formula 



/! F z 25.369 x 50.617 



/2 = f^i /2 = 25.869 + 50.617 = 16 ' 8 " mm ' 

 behind H 2 B, and hence 16.899 + 5.924 = 

 22.823 mm. behind the cornea, and 22.823 

 2.11 = 20.713 mm. behind H 2 of System C. 

 The posterior principal focal distance of the eye 

 is therefore 20.71 mm. 



Parallel rays falling on the posterior surface of 

 the lens are refracted by the lens and converge 

 at a point fa B 50.617 mm. in front of Hi B. 

 They meet the anterior surface of the cornea 

 50.617 - 5.726 = 44.891 mm. from HI B. They 

 are further converged by the cornea to 



23.266 X 44.891 



before the cornea, or 13.75 + 1.75 = 15.5 mm. 

 before H^ C. The anterior principal focal dis- 

 tance of the eye is therefore 15.5 mm. 1 



1 In this discussion I have followed closely, in some places 



