THE tOLARIMETER 73 



up with a semicircular plate of quartz, cut in such a direction 

 in reference to the optic axis of the crystal, and of such a 

 thickness that it retards the light passing through by half a 

 wave length. We thus obtain two beams of polarised hght, 

 differing in phase by half a wave length. At a certain angle, 

 therefore, interference will take place, as explained earlier, in 

 the case of the rays passing through the quartz plate, and one 

 side of the field will appear completely black and the other 

 completely bright. A position can, however, be found when 

 both sides are completely bright or, on the other hand, com- 

 pletely dark. By a differential arrangement of this sort it is 

 much easier to distinguish the alteration of illumination which 

 occurs on moving the analyser, and we can thus make exact 

 observations of the effect upon the polarised ray of substances 

 placed between the two prisms. 



The effect of the quartz plate just referred to is not only 

 to retard the wave by half a length, it also alters its plane of 

 vibration, and, therefore, if such a plate is inserted between 

 crossed Nicols, a certain component of the light passing 

 through the quartz will also pass through the analyser. In 

 order to produce interference and consequent darkness, it is 

 necessary to rotate the analyser through a certain angle in 

 order that the rays passing through the quartz should be 

 brought into the same plane as those passing through the 

 analyser. Other substances besides quartz are capable of 

 altering or rotating the plane of polarisation, even when their 

 solutions are placed in the polarimeter, between the two prisms. 



For observing the effect of such solutions, a glass tube 

 closed by thick glass discs and screw caps is made use of, 

 tubes of different lengths being used according to the 

 concentration of the liquid to be examined. 



In Fig. 15 1 are given the essentials of construction of 

 the Laurent polarimeter. 



' Adapted by permission from Dr. A. Findlay's Practical Physical 

 Chemistry. 



