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observers, and which may generally be adapted to almost any instru- 

 ment. 



In conducting experiments on this subject it is usual to employ 

 one of three methods of producing polarized light, viz. — 



1st. A bundle of plates of crown glass (see PI. XII. fig. 1), from which 

 the light is to be reflected at an angle of 56° from the perpendicular, a 

 portion of the light being reflected from the surfaces of the glass, and 

 another portion being transmitted, each of these portions is found by 

 analysis to consist of rays of light polarized in opposite planes. This 

 method is employed when it is required to illuminate a large object, 

 as in the single or doublet microscope, and in the oxy-hydrogen ap- 

 paratus. 



2nd. A plate of tourmaline (see fig. 2), cut parallel to the crystalline 

 axis of the crystal : it is necessary to be careful in the selection of 

 tourmalines ; they should be as free from colour as possible, and dark- 

 coloured ones to be avoided as almost useless in investigations of 

 colours. Tourmalines are, however, seldom used as polarizers, in 

 consequence of the difficulty of obtaining them of sufficient mag- 

 nitude. 



3rd. A NicoVs or single-image prism (see fig. 3), which is a portion 

 of a crystal of Iceland spar cut and combined with a piece of glass, 

 so as to throw out of the field of view one of the two images produced 

 by the double refraction of the crystal. This is the most eligible for 

 the compound microscope, in being perfectly free from colour, and 

 requiring no adjustment beyond transmitting the light, either direct or 

 reflected from the mirror ; it may be fitted into the diaphragm, and 

 also be adapted to the achromatic condenser, to be employed with 

 high powers in the examination of minute structures. 



As a test of polarized light, it is necessary to employ analyzers of 

 the most convenient form ; they consist of apparatus possessing the 

 same properties as the polarizers, that is, of causing light to pass in 

 only one plane of vibration : the polarizer and analyzer should, when 

 superposed, allow the light to pass freely in one position, and produce 

 a dark field when turned one-fourth of a revolution ; for this purpose 

 either of the two last-named pieces of apparatus can be employed : 

 the tourmaline will be found useful for ascertaining if the object under 

 examination possess the property of polarization, as it may be placed 

 over the eye-piece without cutting off any part of the field of view; 

 but if the analyzer be required as a test of colour, the Nicol's prism 

 should be employed, and placed in the body of the instrument, and as 

 near to the back of the object-glass as possible. 



