POLARISED LIGHT 373 



which rotate the colours in opposite directions, if a plate be 

 composed half of one, and half of the other, of the proper thick- 

 ness, it is a very sensitive test of any optical rotation, as the 

 purple transition -tint seen all over in one position of the 

 analyser, changes towards blue in one half and red in the 

 other, on the least rotation. Such is therefore used to demon- 

 strate the rotation of fluids. 



Another useful and more sensitive bi- quartz preparation 

 is that shown in fig. 205. In the portion B, the wedge B is of 

 say right-handed, and A of left-handed quartz, the effect of 

 which is a black band across the centre where the thicknesses 

 are equal (i.e. in the dark field) and coloured bands on each 

 side. In the half c D the wedges 

 are reversed. The consequence is, 

 that on the least additional rotation 

 in one direction by any substance 

 used with the wedges, or the least 

 rotation of the analyser, the bands 

 move in opposite directions, and 

 the distance the analyser has to 

 be rotated to bring them back, is a 



measure of the rotation due to the PIQ 2 o5.-Bi-quartz wedges 

 substance examined. 



Plates of quartz may be obtained in which both right- and 

 left-handed crystallisation occurs, and such are very beautiful 

 objects. Amethyst is a quartz crystal with the contrary cry- 

 stals arranged in narrow parallel bands, and such are still more 

 beautiful, but a large one is very difficult to find : one that will 

 cover a standard- sized disc is a gorgeous object. Plates about 

 | inch diameter can be obtained without difficulty. Any clean 

 quartz crystal of good size which is a violet colour, is almost 

 certain to exhibit either amethyst or cross-crystallisation, and 

 should be cut up into polariscope specimens. 



Other crystals can be obtained which rotate the beam, 

 and most of them in bi-quartz form. They are, however, all 



