Stereoisomerism. 1 6 3 



is greater than that of the minimum value for the ex- 

 traordinary ray in Iceland spar, but less than that^ of 

 the ordinary ray. If the incidence of the ordinary ray is 

 sufficiently oblique, it is totally reflected. If the plane 

 AC is inclined at a suitable angle, the ordinary ray PO 

 will be incident on the surface AC at an angle greater 

 than the critical angle, and will be totally reflected along 

 GO', whilst the extraordinary ray PEE' will pass through 

 the prism. 



Now this emergent ray has certain properties which 

 distinguish it from ordinary light rays. The ether vibra- 

 tions take place only in one plane, and the light is then 

 said to be " plane polarized." * If this polarized light is 

 allowed to fall on another Nicol prism, it will be found 

 that on rotating the latter, the field of view will appear alter- 

 nately dark and light, the minimum following the maximum 

 as the second prism is rotated through an angle of 90. 

 The prism by means of which the light is polarized is 

 called the " polarizer," whereas the second prism is called 

 the " analyzer." Suppose now that the two prisms are so 

 placed that the maximal amount of light in the field is 

 obtained, and suppose a liquid or a solution of a substance 

 is placed between the analyzer and polarizer. In the case 

 of certain substances, no effect will be observed (except a 

 slight diminution of the intensity). In the case of other 

 substances, however, the field will be darkened, and the 

 maximal intensity of light will only be observed again if 

 the analyzer is rotated. In this case the substance, or the 

 solution of the substance, has rotated the plane of polarized 

 light and is said to be optically active, and the degree of 

 its activity can be measured by ascertaining the angle 

 through which the analyzer must be rotated to produce 



* For fuller details as to the theory of polarization, reference should be 

 made to a text-book on physics. 



