POLARISATION OF LIGHT. 



139 



object-glasses, is called the analyser. The definition is 

 better if the analyser be placed at top of the A eye-piece, 

 and it is more easily rotated than the polariser. 



Method of using the polarising Prism, fig. 83. After 

 having adapted it to slide into a groove on the under-surface 

 of the stage, it is held in its place by turning the small 

 milled-head screw at one end : the other prism, fig. 83#. is 

 screwed on above the object-glasses, and made to pass into 

 the body of the microscope itself. The light having been 

 reflected through them by the mirror, it becomes necessary 

 to make the axes of the two prisms coincide ; this is done 

 by regulating the milled-head screw, until by revolving the 

 polarising prism, the field of view is entirely darkened twice 

 during one revolution. This should be 

 ascertained, and carefully corrected by 

 the maker and adapter of the apparatus, 

 If very minute salts or crystals are to be 

 viewed, it is preferable to place the ana- 

 lyser above the eye-piece; it will then 

 require to be mounted as in fig. 84. Thus 

 the polariscope consists of two parts ; one 

 for polarising, the other for analysing 

 or testing the light. There is no essen- 

 tial difference between the two parts, 

 except what convenience or economy may 

 lead us to adopt ; and either part, there- 

 fore, may be used as polariser or analyser ; 

 but whichever we use as the polariser, the 

 other becomes the analyser. 

 The tourmaline, a precious stone of a neutral or bluish 

 tint, forms an excellent analyser; it should be cut about 

 2\,th of an inch thick, and parallel to its axis. The great 

 objection to it is, that the transmitted polarised beam is 

 more or less coloured. The best tourmaline to choose is 

 the one that stops the most light when its axis is at right 

 angles to that of the polariser, and yet admits the most 

 when in the same plane. It is necessary to choose the 

 etone as perfect as possible, the size is of no importance 

 when used with the microscope. 



In the illumination of objects by polarised light, when 

 under view with high powers, for the purpose of obtaining 



Fig. 84. 



