52 



ELEMENTARY CHEMICAL MICROSCOPY 



A nicol prism consists of a long rhomb of calcite cut length- 

 wise in an oblique plane forming angles of 90 degrees with the 

 upper and lower faces of the rhombs and cemented together 

 again with Canada balsam, see Fig. 22. If a ray of light R enters 

 such a prism it is polarized, being resolved 

 into two component rays vibrating at right 

 angles to each other. One of these rays O, 

 known as the ordinary ray is deflected 

 slightly more than the 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 completely 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 ray emerges 

 parallel to its entrance direction at R. 

 In this position the nicols are said to be 

 ^aralleL_ 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. 23. The arrows indicate the planes of vibration 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. 



FIG. 22. Construction and 

 Path of Light Rays in 

 a Nicol Prism. 



