192 THE MICROSCOPIST. 



touching the obtuse solid angles. The cut surfaces having been 

 carefully polished, were then cemented together with Canada 

 balsam, so as to form a rhomb of nearly the same size and 

 shape as it was before cutting. 



By this arrangement, of the two rays into which a beam of 

 common light would be separated, only one is transmitted, the 

 other being rendered too divergent. Two of these prisms form 

 the usual polarizing apparatus of the microscope, being used in 

 the same manner as the bundles of glass plates, Fig. 62, just 

 described. One of the prisms is adapted to the under surface 

 of the stage, and is called the polarizer ; the other, called the 

 analyzer, is placed above the eye-glass. 



Dr. Brewster recommends that the analyzing prism be placed 

 immediately behind the object-glass, next the eye, having a 

 rotation independent of the body of the microscope. 



Another method of polarizing light, is to disperse or absorb 

 one of the oppositely-polarized beams which constitute common 

 light, and leave the other beam polarized in one plane. These 

 effects may be produced by thin plates of agate, tourmaline, 

 &c. 



Many persons employ a thin plate of tourmaline as an 

 analyzer in place of a Nicol's prism, and if its color be not 

 objectionable, it may be used to advantage, as the field of 

 view is not so much contracted as when a prism is used. A 

 tourmaline of a neutral tint is an excellent analyzer. 



The splendid colors, and systems of colored rings produced 

 by transmitting polarized light through transparent bodies that 

 possess double refraction, are the most brilliant phenomena 

 that can be exhibited. They were discovered simultaneously 

 by M. Arago and Dr. Brewster. 



To see these colors : having the polarizing apparatus so 

 placed that no light can be seen through it, place a thin film 

 ofynica or sulphate of lime (between the twentieth and fiftieth 



