CU. LV.] 



REFRACTION. 



757 



The term index of refraction means the ratio of the sine of the 

 angle of incidence to that of the angle of refraction ; this is 

 shown in the next figure. 



Fig. S77- 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), and 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 perpen- 



dicularsdrawn to N N'; then = index of refraction. 



In this reduced or simplified eye, the principal posterior focus, 

 about 23 mm. behind the spherical surface, would correspond to 



Fig. 578. Diagram of the optical angle. 



the position of the retina behind the anterior surface of cornea. 

 The refracting surface would be situated about midway between 

 the posterior surface of the coruca and the anterior surface of 

 the lens. 



