REFRACTION. 160 



Although the ratio of the sines is constant, the re- 

 fractive index varies in different media. Thus that 

 of air is 1-0003; of water, 1 '336,- of Canada balsam, 

 1*549; of crown glass, from which window-panes are 

 made, 1*535; of flint glass, from which bottles are 

 made, 1*6 ; of Faraday's heavy glass, composed of sili- 

 cated borate of lead, 1*8; and of that consisting of 

 borate of lead, 2*0, 



A knowledge of this " law of the sines " is of prac- 

 tical importance in determining the direction which 

 the rays will pursue when transmitted through glass 

 lenses, &c. the refractive index of which is known; 

 or in ascertaining the curve which should be given to 

 their surfaces for producing a particular refraction 

 and focal length. Thus, supposing the plate of glass 

 in PL I. fig. 2 to consist of crown glass, the refractive 

 index of which is 1*5, the length of the sine of re- 

 fraction, t r, will be equal to one part or dimension, 

 while the sine of incidence, s i } is equal to one part 

 and a half. 



It must be remarked that when light is incident at 

 a right angle to the surface of the medium, no re- 

 fraction takes place, the transmitted ray pursuing its 

 original course. 



When a ray of light leaves a denser medium, such 

 as glass, to enter a rarer medium, such as air, it be- 

 comes refracted from the perpendicular. In such case, 

 the angle of refraction being greater than the angle 

 of incidence, its sine will also be greater than that of 

 the latter ; but the ratio is still preserved. 



Reflexion. When rays of light fall upon a plane 

 surface, as the flat surface of the mirror, a greater or 

 less number of them are reflected, and this accord- 

 ing to a definite law, by which the angle of incidence, 

 or that formed by the incident ray with the perpen- 

 dicular, is equal to the angle of reflexion, or that 

 formed by the reflected ray with the same. Thus, as 

 shown in PL XII. fig. 3, the angle i bp, formed by 



Q 



