REFRACTION IN THE EYE 459 



cornea in a point about 10 mm. behind the retina. 

 Thus the eye is reduced to a single refracting 

 surface, the cornea, separating two media, the air 

 and the vitreous humor. The index of refraction 

 of these media is f . The principal focal distances 

 are proportional to the coefficients of refraction 

 of the first and last media ; F\ is 15 mm. and F 2 

 20 mm., measured from the principal point. The 

 visual axis (from the cornea to the retina of the 

 reduced eye) is therefore 20 mm. In order to 

 bring parallel rays to a focus at 20 mm., the 

 index of refraction being J, the radius of curva- 

 ture of the cornea of the reduced eye should be 

 5 mm. 



In such a reduced eye the retinal images have 

 the same position and size as in the ordinary 

 eye. The reduced eye is shown in normal size 

 in Fig. 65. 



Fig. 65. The reduced eye. Normal size (bonders). 



K is the optical centre or nodal point. 

 h, the principal point. 



Kh 5 mm., the radius of curvature of the 

 refracting surface. 



