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A Simple Form of Selenite Stage. By W. Hislop, F.R.A.S. 



(Bead Nov. 26th, 1869.) 



A short time since I endeavoured to point out the necessity of a 

 closer attention to the mechanical arrangement of the polariscope 

 as applied to the microscope, in order to render the advantages of 

 illumination by polarized light, more generally available. As a 

 contribution to this end, I described a piece of apparatus which 

 I had contrived, used, and exhibited for some time, and which I 

 called an Analyzing Selenite Stage ; the arrangements of which 

 were so contrived as to contain three separate films of selenite, and 

 to give them the power of rotation in all directions, together or 

 separately, and in close contact with the object under examination. 

 The facility with which this could be done, and the proper effect 

 produced and registered for each object was also pointed out. 



Since then the use of that instrument has produced some curious 

 results, and the simple arrangement which I have now to describe 

 is one of them, and will, I think, greatly increase the range of the 

 ordinary forms of polarizing apparatus, without the necessity of 

 incurring the expense of more elaborate contrivances. 



I merely avail myself of a previously known fact — namely, that if 

 a film of mica be superimposed on a film of selenite, the colour 

 transmitted by the latter will be much modified. I find that if we 

 take a film of selenite producing, say, a blue colour, and if we 

 superimpose on that a film of mica, of say '002 in thickness, and 

 separate their axes of polarization by a certain angle, we obtain by the 

 combination nearly all the colours which we have got by different 

 films of selenite. I cannot fix the exact angle of divergence, as it 

 varies with different specimens of mica, but it is easily ascertained 

 by trial. 



To make this compound film available, I use first an ordinary brass 

 slide about 3 inches by 1 1 of an-inch (See Fig.), with a ledge at the 



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