546 



THEORY OF COLORS. 



another direction, n E, so that an eye placed at E would see the object in the 

 direction E n. 



This deflection which a ray of light suffers in passing from one transparent 

 medium into another, having a different density, is called refraction. 



REFRACTION AT PLANE SURFACES. 



Let S S', fig. 2, represent the surface which separates two transparent media. 

 P being less dense than P 0'. Let A P be a ray of light falling at P, and let 

 0' be perpendicular to S S'. After passing into the denser medium the r.iy 

 will follow the course P A', making with the perpendicular P 0, a less angle 

 than A P O. 



Fig. 2. 



If, on the other hand, the ray passed from A' to P, it would follow the course 

 P A in the less dense medium. This law of refraction is usually expressed 

 thus : when light passes from a rare into a dense medium, as from air to water, 

 or from water to glass, it is always deflected toward the perpendicular to the 

 reflecting surface, and when it passes from a denser medium into a rarer, as 

 from glass to water, or from water to air, it is bent from the perpendicular. 



The extent of this deflection has been determined by a general law, which, 

 expressed in the language of geometry, is, that the sine of the angle of inci- 

 dence bears to the sine of the angle of refraction, a fixed ratio when the media 

 are given. 



From this it follows that the deflection of light by refraction will always be 

 increased with the obliquity of the incident rays. 



It is also found that the degree of refraction will be greater the greater the 

 difference of the density of the media is. Thus the refraction is greater when 

 a ray passes from air into glass than when it passes from air into water ; it is, 

 also, greater when it passes from glass into air than from glass into water. 



In his celebrated optical investigations, Newton found that the solar beam 

 was composed of different kinds of light, which, besides differing in color, also 

 differ in refrangibility, that is to say, if they fall at the same angle on any re- 

 flecting surface, they will not pass in the same direction through it, but will 

 follow different directions, according to their different susceptibilities of being 

 refracted. 



The kind of experiment by which this remarkable fact was ascertained is as 

 follows : 



Suppose a beam of light proceeding from the sun to enter a hole in a win- 

 dow-shutter and to fall obliquely on the surface of a triangular piece of glass, 



