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THE POLARISCOPE, AND HOW TO WORK WITH IT. 
By C. HOCKIN, M.A. 
T he object of this article is to describe and to attempt an 
elementary explanation of the more ordinary phenomena 
produced by the polariscope. The results that are to be ob- 
tained with this little instrument, with very little trouble, are 
exceedingly beautiful. Perhaps an idea of the difficulty of 
understanding the subject has prevented the polariscope being 
so popular as many other scientific instruments. Of course, 
much labour and knowledge is required to study successfully 
the laws of the motion of light in crystalline bodies, and the 
relation of these laws to the shape of crystals. But few, we 
think, who possess a micro-polariscope appreciate its value. 
If we can awaken the curiosity of such to further research, we 
shall have attained our object, and we think they will be amply 
rewarded for their trouble by the beauty of the results obtained. 
A figure of a very convenient form of polariscope, known as 
Niirenberg’s, is given at fig. 1, PI. XXV. ab is a plate or plates 
of clear unsilvered glass, supported, like an ordinary mirror, by 
two pins L and M, which fit rather firmly into circular holes in the 
pillars CD, EF. An index and sector at l enables the plate ab to 
be set at any angle with the vertical. The feet of the pillars cd, 
EF are fixed in a circular base, and between them is a round 
horizontal mirror. On the top of CD, ef is the annular frame 
of wood Rs. In this frame turns another ring of brass, and 
supporting the pillars gh, ki. pq is a plate of glass darkened 
at the back with some unreflecting substance, as lampblack. 
AB is called the polarizer, pq the analyser. A thin glass plate 
covering the aperture between h and k serves as a stage to 
support the object to be examined. The plates pq, ab are 
ordinarily set to make an angle of 33:^° with the vertical. A 
ray of light coming in the direction x T will strike A b at an 
angle of 56|°, will be reflected down to the mirror z, then, 
passing vertically back through the plate AB to PQ, will finally 
be reflected in the direction uv (uv being a line in the same 
vertical plane as zu, and making, with pq, the same angle as 
