PHYSIOLOGY OP VISION. 167 



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. 



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 towards a perpendicular line drawn 

 from the surface of the medium. But when the ray 

 passes out of a denser into a rarer medium, the refrac- 

 tion isfiom the same perpendicular. 



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

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



Fig. 101. 



pursuing its original course along the line, s, o, is refrac- 

 ted into the direction s, t, which is a line situated 

 between s, o, and s,p ; this latter line being drawn per- 

 pendicular 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 con- 

 trary direction ; that is, it inclines towards the perpen- 

 dicular line t,i, drawn from t, within the denser medium, 

 and describes the new course t, u, instead of t, v. 



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. 



In the application of these principles to the form of a 

 dense medium, which shall bring the rays of light pass- 



What is the direction when it passes into a rarer medium 1 What 

 does the amount of refraction correspond with 1 



