Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
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 
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 
I 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 
\ B 
\ I 
A \ 
Fig. 205.—Bi-quartz Wedges


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