POLARISATION. 



brass plates, between which the object-slide is inserted so that 

 an object lying thereon is nearly in the axis of rotation, and 

 consequently suffers little or no lateral displacement on rotation. 



FIG. 176. 



The indicator P, connected with the milled head D, moves along 

 the graduated scale on rotation, and thus gives the reading of the 



FIG. 177. 



angle. 



It is advisable, for certain purposes, to arrange the ap- 

 paratus so that the object can be turned under water or other 

 liquid. This is the case, for instance, in the trough apparatus of 



