MOUNTING OF MICROSCOPES. 
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' V c'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 k! c' and b' b' 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 have, 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. 
Various 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- OE 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. 
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