156 J. IJ. HARFORD — POLARISATION OF LIGHT. 



fractive power of tlic spav upon the lower power of the balsam at 

 so slight an angle that it is, in accordance with a well-known law, 

 which time will not allow of explaining here, incapable of pene- 

 trating it, and is wholly reflected. Thus, for optical piirposos, and 

 the microscope especially, a provision is secured for obtaining a pure 

 beam of polarised light, i.e. the beam that issues from the crystal. 



In reference to the doubly refracting power of the oblique and 

 prismatic class of crystals, it is further to be observed that in the 

 great majority of cases the separation of the two rays (ordinaiy 

 and extraordinary) is so slight as to be altogether imperceptible to 

 common observation. It is detected and proved, however, by other 

 means, chiefly by the remarkable elfects of colour, which will be 

 treated of in a subsequent part of this lecture. 



There is one other mode in which light may become polarised, 

 and that is by what is called absorption. Take a thin slice of the 

 mineral tourmaline, cut in a direction parallel to the axis, and it is 

 found that the light passing through this plate is polarised, and its 

 property can be proved in the usual modes. Take a second similar 

 cutting, and it is observed that when both plates are placed over 

 each other in the same direction as their position in the natural 

 crystal — or one of them in the position vertical to it — the light 

 passes tlirough both with no alteration beyond what is due to the 

 diminution of light owing to the increased thickness. Place, how- 

 ever, the two plates across each other at a relative position rect- 

 angular to the former, and the light is wholly stopped. In this 

 instance it is to be remarked that the tourmaline belongs to the 

 class of prismatic forms ; that in this case double refraction occurs 

 in all directions except along the axis ; and that double refraction 

 ought to occur, therefore, in the passage of light through the platt s 

 just referred to. It does so, but one of the rays is obstructed, 

 suppressed, or absorbed in its passage ; and one only, and that 

 polarised, emerges. 



All the foregoing considerations seem to point towards tlio first 

 step of the theory which is adopted in explanation. AN'ithout 

 attempting a lengthened dissertation, it may briefly be stated thus : 

 light consists of, is caused by, or accompanied by, whichever term 

 may be most appro])riatc or intelligible, vibrations— oscillations — 

 movements to and fro — of particles of a highly clastic fluid nunlium 

 called the " luminiferous ether." These vibrations take place in 

 directions transverse to the line of direction of the propagation of 

 the light. The law under which tliese vibrations combine to pro- 

 duce what are termed waves or undulations will be considered later. 

 In ordinary light these vibrations occur in all directions round tlie 

 lino of progression of light. In polarised liglit, on the contrary, 

 they are assorted into definite directions, and in the case of a ray 

 divided into two, either by ordinary or double refraction, the 

 arrangement of the vibrations is such that the pianos of these 

 vibrations are at right angles to each otlicr. A little tliouglit will 

 8uffic:e to make tliis theory apply to the several cases of polarisation 

 already noticed. 



