778 



THE EYE AND VISION 



[CH. LVL 



the curvature of each surface are known. These data are as 

 follows : 



Index of refraction of cornea . . . . = 1'37 



,, ,, aqueous and vitreous . 1'34 to 1'36 



lens 



_ /I '4 in outer to 1 '45 

 \ in inner part. 



Radius of curvature of cornea . . . . = 7 *8 mm. 



,, anterior surface of lens == 10 



,, ,, posterior . . . = 6 ,, 

 Distance from anterior surface of cornea to 



anterior surface of lens . . . . -- 3'6 ,, 

 Distance from posterior surface of cornea to 



posterior surface of lens . . . . = 7 '2 ,, 

 Distance from posterior surface of lens to 



retina ... , = 15 -0 , 



With these data it has been found comparatively easy to reduce 

 by calculation the different surfaces of different curvature into one 



N 



Pio. 579. If P P is a line which separates two media, the lower one being the denser, and A O is a ray 

 of light falling on it, it is bent at O towards the normal or perpendicular line N N'. A O is called 

 the incident ray, and O B the refracted ray ; A O N is called the angle of incidence (i), N' O B the 

 angle of refraction (r). If any distance O X is measured off along O A, and an equal distance O X ' 



along O B and perpendiculars drawn to N N'; then 7>= index of refraction. 



mean curved surface of known curvature, and the differently refract- 

 ing media into one mean medium the refractive power of which is 

 known. 



The simplest so-called schematic eye formed upon this principle, 



