PHYSIOLOGY OF VISION. 163 



refracted out of a straight line, unless it strikes the new 

 medium in a perpendicular direction. Air, water, glass, 

 or any other substance through which light passes, is 

 called a medium. 



491. If the ray passes obliquely, from a rarer into a 

 denser medium, as from air into water, or from water into 

 glass, it is refracted toward a perpendicular line drawn 

 from the surface of the medium. But when the ray passes 

 out of a denser into a rarer medium, the refraction is from 

 the same perpendicular. 



492. Thus the ray e, Fig. 101, striking obliquely on the 

 surface of a denser medium, at the point s, instead of pur- 

 Fig. 101. 



suing its original course along the line s, o, is refracted 

 into the direction s, t, which is a line situated between s, o, 

 and s, p ; this latter line being drawn perpendicular to 

 the surface of the medium, at which the ray enters. When 

 the ray arrives at t, it passes from a denser into a rarer 

 medium, and is refracted in the contrary direction ; that is, 

 it inclines toward the perpendicular line t, i, drawn from 

 t, within the denser medium, and describes the new course 

 t, u, instead of t, v. 



493. In all cases of refraction, the amount corresponds 

 to the degree of obliquity of the ray to the surface which 

 refracts it ; while that ray which passes perpendicularly 

 from one medium to another, no matter how different their 

 densities, is not refracted at all, but pursues a straight course, 

 as though the media were of one and the same density. 



494. In the application of these principles to the form 

 of a dense medium, which shall bring the rays of light 



What is the direction when it passes into a rarer medium ? What does 

 the amount of refraction correspond with ? 



