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 vh 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 vh of the first, the 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. 23. The arrows indicate the planes of vibration in the 
direction of the short diagonal.^ 
^ 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. 
