2 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



large or small, its sine e e' bears a constant ratio in each case to 

 the sine w w' or g g' or d d', of the angle of refraction ; and this 

 ratio is termed the ' index of refraction.' 



The ' index of refraction ' is determined for different media, by 

 the amount of the refractive influence which they exert upon rays 

 passing into them, not from air, but from a vacuum; and in ex- 

 pressing it, the sine of the angle of refraction is considered as the 

 unit, to which that of the angle of incidence bears a fixed relation. 

 Thus when we say that the 'index of refraction' of Water is 1*336, 

 we mean that the sine e e' of the angle of incidence e o c of a ray 



passing into water from a vacuum, is to the sine w to' of the angle 

 of refraction w o c', as 1*336 to 1, or almost exactly as lg to 1, or 

 as 4 to 3. So, again, the index of refraction for (flint) Glass, being 

 about 1 '6, we mean that the sine e e' of the angle of incidence of a 

 ray e c passing into glass from a vacuum, is to the sine g g' of 

 the angle of refraction g c', as 1*6 to 1, or as S to 5. So in the 

 case of Diamond, the sine e e' is to the sine d d' as 2*439 to 1, or 

 almost exactly as 2^ to 1, or as 5 to 2. Thus, the angle of inci- 

 dence being given, the angle of refraction may be always found by 

 dividing the sine of the former by the index of refraction, which 



