MICRO-POLARISCOPE. 



sented to the reflecting surface. If a polarised ray, however, fall 

 at a certain inclination upon the same surface, it will be reflected 

 or absorbed according to the side of it which is turned towards 

 the reflecting surface. Thus, if the side A' B' or c' D' be pre- 

 sented towards the reflecting surface, the ray will be reflected as 

 if it were common light, but if the side B' c' or A' D' be turned 

 towards the reflecting surface, it will not be reflected at all, but 

 will be, as it were, smothered or extinguished. 



The sides A' B' and c' D', which are opposite to each other, 

 have, therefore, a property contrary to that of the sides B' c' and 

 A' D', so that they are respectively called the poles of the ray, 

 just as the ends of a voltaic circuit having contrary electric pro- 

 perties are called the positive and negative poles of the voltaic 

 battery, and the ends of a magnet are called its boreal and austral, 

 or south and north poles. 



The effects which polarised light produces when it falls upon, 

 or is transmitted through, various substances, more especially 

 such as are in the state of crystallisation, are of the highest 

 physical importance, being in most cases the indication of mole- 

 cular and other properties, by which optics has been placed in 

 relation with, and has become the handmaid of, almost every 

 other branch of physical science. 



71. There are various expedients by which a ray of common 

 light can be polarised. It will be polarised if it be reflected at 

 a certain inclination, called from that circumstance the angle of 

 polarisation, from certain surfaces. Each substance has its own 

 angle of polarisation. That of glass, for example, is 3o^. It 

 is also polarised if it pass through certain transparent crystals. 

 Some of these, while they polarise the ray, split it into two, both 

 being polarised, but in planes at right angles to each other ; that 

 is, for example, the sides A' B' and c' n f being white in one, and 

 black in the other. 



The well-known mineral called Iceland spar is an example of 

 this class of crystals. 



Such crystals are called double-refracting crystals, because the 

 two rays into which the ray of common light is split are refracted 

 by the crystal in different directions, and according to different 

 laws. 



"When a polarised ray is transmitted through such a crystal, 

 according to certain conditions, it will either pass through it, as 

 it would through any ordinary transparent medium, or will be 

 extinguished by it, according to the side of the ray to which 

 certain faces of the crystal are presented. Such crystal is related 

 to the poles of the ray, therefore, in the same manner as the 

 reflecting surface already described. 



F 2 67 



