MOUNTING OP MICKOSCOPES. 



the prism, the image will gradually re-appear, at first faintly, 

 and by degrees brighter, until the prism is moved through 

 another quarter of a revolution, when the image will be again 

 seen fully illuminated. Like changes will take place during the 

 other two quarters of a revolution. 



Similar effects will be produced if the prism p' be fixed, and 

 P be turned round its axis. In this case, by moving the polar- 

 ising prism P round its axis, the polarised ray is made to revolve, 

 because the position of its poles a' b' c r d' has always a fixed rela- 

 tion to the faces of the prism p. Since, therefore, the polarised 

 ray revolves, it presents successively all its sides to the prism p', 

 by which it is accordingly alternately transmitted, and absorbed 

 wholly or partially in the same manner, exactly as if the ray were 

 fixed, and the prism p' carried round it. 



By the appearance and disappearance of the image correspond- 

 ing with the position of the prism p', the position or direction of 

 the planes of polarisation A' c' and B' D' of the polarised ray is 

 known. 



These effects will be produced if the objects through which the 

 light is transmitted or by which it is reflected have themselves 

 no polarising influence. But if they h^re, various other pheno- 

 mena will ensue, depending on the character and degree of that 

 influence ; but whatever it be, the state of the light, which pro- 

 ceeding from the object-glass forms the image, will be ascertained 

 by the prism P', which is consequently called the analysing prism, 

 the other P being denominated the polarising prism. 



Yarious physical characters are thus discovered in the objects 

 submitted to the microscope by determining the optical effects 

 they produce on polarised light, and many striking and beautiful 

 phenomena are developed. 



THE MOUNTING OP MICROSCOPES. 



74. The methods of mounting microscopes, so as to adapt them 

 to the convenience and the ease of observers, are very various, 

 depending on the purposes to which they are applied, their price, 

 the exigencies of the purchaser, and the skill, taste, and address 

 of the maker. 



The qualities which it is desirable to confer upon the stand and 

 mounting of the instrument are simplicity of construction, easy 

 portability, smoothness and precision in the action of all the 

 moving parts, and such combinations as may cause any tremor 

 imparted to the stand to be distributed equally over every part of 

 the mounting. These capital objects are attained very com- 

 pletely in all the mountings of the best makers, British and 

 Foreign. 



