22 REFLEXION AND REFRACTION. 



cular to the bounding surface. On the contrary, when the 

 ray passes from a denser into a rarer medium, the angle of 

 incidence is less than the angle of refraction, and the devia- 

 tion is from the perpendicular. 



(26) The angles of incidence and refraction are in the same 

 plane ; and their sines are in an invariable ratio. 



In order to verify this law experimentally, it is only neces- 

 sary to measure several angles of incidence at the surface of 

 the same medium, and the corresponding angles of refraction. 

 This was done by Ptolemy in the second century, and sub- 

 sequently by Vitello in in the thirteenth ; but both of these 

 observers failed in discovering the connecting law. The law 

 of refraction, just stated, was discovered by Willebrord Snell, 

 about the year 1621. 



If u and v be employed to denote the angles which the 

 portions of the ray in the rarer and denser medium, respec- 

 tively, make with the perpendicular to the common surface, 

 the second part of the law of refraction is expressed by the 



equation, 



sin u = n sin 0, 



JJL being a constant quantity. This constant is termed the 

 index of refraction ; and since u > v, it is always greater than 

 unity. 



"When a ray of light passes into any medium from a va- 

 cuum, the index of refraction is in that case termed the abso- 

 lute index of the medium. For air, and the gases, it exceeds 

 unity by a very small fraction ; for water, fi = 1*336 ; for 

 crown glass, /m = 1*535; for diamond, ju = 2*487; and, for 

 chromate of lead, /m = 3. 



(27) When a ray of light traverses a medium bounded by 

 parallel planes, and re-enters the original medium, the emer- 

 gent ray is parallel to the incident. 



