POLARISED LIGHT 351 



and the other black ; but on rotating the crystal, this will 

 reverse as at B ; or it will be also reversed by rotating the 

 prisni to a position at 90 angle with the first. 



201. Piles of Glass. The effect of these is most conveni- 

 ently demonstrated by mounting a pile of from twelve to twenty 

 plates, B c (fig. 198), of the thinnest and most colourless glass 

 that can be procured, in a frame screwed to the edges B c of 

 two triangular pieces of board BCD, which are screwed to a 

 base-board B D, and whose angles are such that a horizontal ray 

 E F striking on the glass, is reflected at the polarising angle to 

 G, while the unreflected portion (i.e. of common light) is trans- 

 mitted to H. The thin glass analyser (fig. 188) would answer 

 the same purpose, but its arrangements cannot be seen in the 

 same way, and moreover it should be used with the present 

 pile. First placing A 



the glass pile in the 

 position of the figure, 



the base- boards Don E 

 atable-stand, and the 



rotating tourmaline F m-Giass pile 



in the stage of the 



optical front, it will be shown that when in one position the 

 tourmaline image is black on the screen by transmitted light, 

 and transparent on the ceiling or overhead screen by reflected 

 light; and that rotating the tourmaline 90 reverses this. 

 Rotating the pile 90 (easily done by laying the pile on its 

 side BCD) the images are also reversed. Using an aperture 

 and the double-image prism, in the same way, the pair of 

 discs will be alternately light and dark according to the 

 positions ; and then substituting the Nicol, it will be shown 

 how one half the phenomena are suppressed or turned aside 

 by this piece of apparatus. Finally, substituting the glass 

 analyser (fig. 188) still with the aperture in the stage, it will 

 be seen how the transmitted beam is either reflected or again 



