PART VII. 

 THE PHENOMENA OF POLAKISATION. 



I. 

 ARRANGEMENT OF THE POLARISING MICROSCOPE. 



IN the examination of microscopic objects in polarised light a suitable 

 arrangement of the polarising Microscope is of the first importance. 

 We shall assume that our readers are aware that this instrument 

 must possess two polarising contrivances a so-called polariser 

 beneath the object, and an analyser above it. We have to explain 

 how these pieces of apparatus may be most serviceably contrived, 

 and how they act under given circumstances ; and we shall inquire 

 what additional apparatus, &c., is desirable for convenience and 

 accuracy in experimenting. Before we consider these points, 

 however, it should be observed that in the following explanations 

 we confine ourselves to actual Microscopes that is to say, instru- 

 ments which can be used for ordinary investigations after the 

 polarising apparatus has been removed. Polarising apparatus, 

 such as the so-called Noerrenberg's polarising Microscope, shall be 

 treated of separately, so far as microscopic objects are concerned 

 therewith. 



1. THE POLARISER. 



If the polariser is required for the higher amplifications, it must 

 provide a cone of light whose peripheral rays deviate at least 8 to 

 10 degrees from the vertical ; for since only half of the incident 

 light is transmitted, a greater convergence is necessary for the 



