REFRACTION IN THE EYE 449 



entering ray is directed. Thus the point in 

 the second principal surface from which the 

 emergent ray appears to come is the same dis- 

 tance from the axis as the corresponding point 

 in the first principal surface to which the enter- 

 ing ray appears to pass. In short, each principal 

 surface is the image of the other, and is of equal 

 size. 



To determine the position of the principal sur- 

 faces there must be found between the two 

 refracting surfaces a point, s, at which an object 

 will form similar images with each refracting 

 surface. These images being similar are images 

 of each other and of equal size. The planes in 

 which they lie are the principal surfaces. 



The Point a. The point s lies between the 

 two refracting surfaces at distances proportional 

 to the principal focal distance of each. It will 

 be remembered that the point o was found by 

 dividing the distance between the two refracting 

 surfaces into two parts, proportional to the radii 

 of curvature of the two surfaces. In System B 

 the two refracting surfaces are the anterior and 

 posterior surfaces of the crystalline lens, which 

 is bounded by the aqueous and vitreous humors, 

 media of equal refractive power. The focal dis- 

 tances of the refracting surfaces are in this case 

 proportional to the radii of curvature. Thus the 



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