On Mica Films and Prisms for Polarizing-Purposes. 109 
an angle of 180° in its own plane. The arrangement of the 
experiment described is further worth notice, since it shows 
at one glance the changes which take place in the six direc- 
tions to which special attention has been directed. The result 
obtained moreover gave occasion for further electrical experi- 
ments with quartz, the results of which I hope to describe in 
a further communication*. 
It is, I think, unnecessary to detail further electro-optical 
experiments with quartz, since all the phenomena I have 
hitherto observed may be deduced from the piezoelectric 
behaviour. How far the hypotheses which form the starting- 
point of my investigation will hold good in all cases yet 
remains to be tested by calculation. 
XYI. Mica Films and Prisms for Polarizing-Purposes. 
By Lewis Wbight. 
To the Editors of the Philosophical Magazine and Journal. 
Gentlemen, 
THE letter of Mr. H. Gr. Madan leads me to hope that some 
interest may be taken in the construction of mica 
designs and combinations; and I therefore give a few hints 
from an experience which has been rather tiresome. I also 
have used the photographic films spoken of; but, except for a 
few special quarter-wave plates, these would soon be ex- 
pensive, and the carte-de-visite size are not large enough for 
work which requires many squares of equal thickness. More- 
over it is veiy tedious to find and mark the " axis " on each 
separate film ; I much prefer to select a fairly large slab, of 
the best quality obtainable, which will provide a quantity at 
a tithe of the cost of the films, and quite as clear. This 
should first be split into moderately thick sheets, say two to 
four waves in thickness, from which the films can be split 
afterwards ; they do not split so evenly direct from the thick 
slab. The process described for splitting would, by its tedium, 
deter many from doing much with mica films ; and it is not 
necessary. The edge being sprung with the point of a sharp 
needle, the sheet is laid on a perfectly flat surface, and the 
film separated by a thin and smooth paper-cutter of ivory or 
tortoise-shell. This is pushed and " coaxed " in gently, 
when, in most cases, the film will come evenly off. The main 
secret is to have a flat surface to work upon, and to keep the 
* I have commissioned Messrs. Steeg and Keuter, in Horuburg voi- 
der Hohe, to prepare quartz suitable for electro-optical experiments, which, 
if desired, can be examined under my direction before delivery. 
Phil. Mag. S. 5. Vol. 16. No. 98. August 1883. K 
