CRYSTALS UNDER THE MICROSCOPE 



161 



other and strikes the balsam cement at such an angle as to be 

 totally reflected; the other ray called the extraordinary ray, 

 passes through the prism and emerges com- 

 pletely polarized. In the diagram at S is 

 shown a cross section of the rhomb. The 

 direction vb through a shorter diameter of 

 the prism rhomb is the plane or direction 

 of vibration of the nicol. If, after emerging 

 from the first prism, the extraordinary ray 

 be sent into a second nicol so placed that 

 its plane of vibration is coincident with or 

 parallel to the direction vb of the first, the 



FIG. 101. Construction 

 and Path of Light Rays 

 in a Nicol's Prism. 



FIG. 102. Position of the Prisms with 

 Nicols Crossed. 



ray emerges parallel to its entrance direction at R. In this 

 position the nicols are said to be parallel. But if the second 

 nicol be turned through 90 degrees, thus taking a position such 

 that its plane of vibration intersects that of the first at 90 

 degrees, the extraordinary ray will behave as though it were the 

 ordinary ray and is completely turned aside. No light emerges 

 from the upper nicol. In this position the nicols are said to 

 be crossed, see Fig. 102. The arrows indicate the planes of vi- 

 bration in the direction of the short diagonal. 1 



1 In the newer polarizing microscopes, the prisms often do not have a rhombic 

 cross section and therefore their planes of vibration do not fall in the direction of a 

 short diagonal. The position of the planes of vibration must then be ascertained 

 experimentally; see Weinschenk, Das polarizations Mikroskop. 



