108 THE MICROSCOPE. 



Metfwd of using tlte polarising Prism, fig. 73. After haying adapted 

 it to slide into a groove on the under-surface of the stage, it is held in 

 its place by turning the small railled-head screw at one end : the other 

 prism, fig. 74, 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 ascer- 

 tained, and carefully corrected by the maker and adapter of the appara- 

 tus. If very minute salts or crystals are to be viewed, it is preferable 

 to place the polariser above the eye-piece ; it will then require to be 

 mounted as in fig. 75. Thus the polariscope consists of two parts ; one 

 for polarising, the other for analysing or testing the 

 light. There is no essential difference between the 

 two parts, except what convenience or economy may 

 lead us to adopt j and either part, therefore, 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 ^V^h of an inch thick, and parallel to its axis. 

 The great objection to it is, that the transmitted polar- 

 ised beam is more or less coloured. The best tourma- 

 line to choose is the one that stops the most light when 

 "^ IT""" its axis is at right angles to that of the polariser, and 



yet admits the most when in the same plane. 

 In the illumination of objects by polarised light, when under view 

 with high powers, for the purpose of obtaining the maximum effect, it 

 is alpo requisite that the angle of aperture of the polariser should be 

 the same as the object-glass, each ray of which should be directly op- 

 posed by a ray of polarised light. The Polarising Condenser is merely 

 an ordinary achromatic condenser .of large aperture, close under the 

 bottom lens of which is placed a plate of tourmaline, used in combina- 

 tion with a superposed film of selenite or not, as required. The effect 

 of this arrangement on some objects is very remarkable, bringing out 

 strongly colours which are almost invisible by the usual mode. 



The production of colour by polarised light has been most clearly 

 and comprehensively explained by Mr. Woodward, in his " Introduc- 

 tion to the Study of Polarised Light."* To render the diagram more 

 * Mr. Woodward constructed a very available form of polariscope for 'most pur- 



