CH. V.] 



POLARIMETER. 



145 



illumination is obtained. If now, instead of water, the 

 tube be filled with a solution of glucose or of certain other 

 substances, it will be found that the illumination is not 

 minimal. To attain this result the analyser must be 

 rotated through a certain angle. The reason for this is 

 that in passing through the glucose solution the plane 



of polarisation has been gradually 

 rotated so that on emerging and 

 striking the analyser some of the 

 light can get through. To get the 

 minimum illumination the analyser 

 has to be rotated to the right through 

 an angle equal to that through which 

 the sugar solution has rotated the 

 plane of polarisation of the ray that 

 entered it. 



The simple arrangement de- 

 scribed is not sufficiently sensitive. 



O(D 



a 



Fig. 22. Plan of a three- 

 field polarimeter. A is 

 the polarising Nicol ; 

 B and C are the small 

 accessory Nicols that 

 resolve the field into 

 three parts, D, E, and 

 F. 



Fig. 23. The appearances seen in a and b indi- 

 cate that the analyser is not in the correct 

 position. When the analyser is correctly 

 adjusted the three parts of the field have 

 an equal feeble illumination as shewn in c. 



Modern instruments have two small Nicol's prisms 

 placed between the polariser and the solution (see fig. 

 22), the effect being to divide the field into three 

 vertical sections. The zero and end points are obtained 

 when the three fields have an equal feeble illumination 

 (fig. 23, c). The source of illumination must be mono- 

 chromatic, since the angle of rotation varies with the wave 

 length employed. That generally used is sodium light, 



