THE POLARISER. 



305 



illumination of the field of view. Such a cone of light may be 

 produced in two ways : either it is provided directly by the 

 polariser, since this apparatus is capable of polarising rays of 

 different inclination equally completely; or the emerging polar- 

 ised rays being approximately parallel, are subsequently made to 

 converge by a convex lens applied between the object and the 

 polariser. The latter arrangement is found in the so-called 

 polariscopes as manufactured by Dove, Noerrenberg, and others ; it 

 is in all cases applicable where there is room for inserting a 

 condenser above the reflecting or refracting contrivances, and 

 hence, as was previously said of illumination in general, does not 

 require any further explanation. Hartnack, Merz, and others 

 have in recent times applied a condensing lens also to the 

 polarising Microscope. Polarisers are in general use which 

 furnish completely polarised light with sufficient convergence 

 without condensers. 



The best known polarising contrivance of this kind is NicoVs 

 prism. We may assume its construction to be known to the 

 student, as it is described in every text-book 

 of Physics ; we will merely add by way of illus- 

 tration that in Fig. 170 a b and ef represent the 

 two end faces which are artificially cut to an 

 angle of 68 to the edge ad ; while in be and de we 

 still see indicated the faces of an obtuse rhombo- 

 hedron. The dividing face b e, where the two 

 halves are cemented together, forms with the end 

 faces angles of 89 17', which may therefore be 

 considered right angles. All these faces are vertical 

 to the principal section of the crystal coinciding 

 with the plane of the paper. 



The optical action of Nicol's prism is as fol- 

 lows : The ray g n, incident from below, divides 

 into two rays, of which the ordinary ray n k 

 undergoes total reflexion at the layer of Canada balsam, and is 

 absorbed at the blackened lateral face of the prism, while, on the 

 other hand, the less refracted extraordinary ray passes through and 

 reaches the field of view in the direction p i. Both rays are polar- 

 ised, the effective extraordinary ray perpendicularly to the plane 

 of the principal section ; its plane of polarisation is called the 

 polarising plane of the Nicol. In the cross- view of the prism it is 



FIG. 170. 



