144 



THE CARBOHYDRATES. 



[CH. V 



A beam of light entering parallel to the long sides of the 

 prism is resolved into its two component rays. The more 

 refracted (ordinary) ray (O) meets the film of Canada 



c D 



Fig. 20. Diagram of refraction in a Ni col's prism. 



balsam (CB), and is completely reflected and absorbed by 

 the black varnish usually placed on the sides of the prism. 

 The other component (the extraordinary ray) (E) passes 

 through the film of balsam and emerges in a polarized 

 condition from the end surface of the' Nicol. 



Fig. 21. Plan of arrangement of a simple polarimeter. 



In a polarimeter (fig. 2 1 ) a second Nicol prism called an 

 analyser (A) is used in addition. This is mounted in such a 

 way that it can be rotated around its long axis. The 

 polarized ray that emerges from the polarising Nicol (P) 

 falls on the face of the analyser, and will only pass through 

 unimpeded provided that it can contrive to vibrate in the 

 same plane. In this position the Nicols are said to be 

 parallel. If the analyser be rotated through an angle of 

 45 the ray is completely absorbed and the Nicols are said 

 to be crossed. On rotating through a further angle K of 45 

 the Nicols are again parallel. Suppose a tube of water be 

 interposed between the two Nicols (fig. 2 1 ) and a source of 

 light at L be viewed through the system, the Nicols are 

 crossed when the analyser is rotated so that the minimum 



