19 



A NEW FORM OF POLARISER. 



By Edward M. Nelson, F.K.M.S. 



{Read January lith, 1919.) 



Figure in Text. 



It is now no longer possible to add a large Nicol to one's box 

 of microscopic apparatus, because of the greatly increased price 

 of large pieces of calcite. A large Nicol is required when the 

 back lens of a low-power condenser has to be filled with light. 

 With the Nicols that can now be obtained, only a condenser with 

 a small back lens can be filled, and if that condenser has any 

 useful aperture it follows that it must be of high power. A 

 condenser suitable for the bulk of microscopical polariscope work, 

 such as rock sections, rings or brushes in quartz, etc., would 

 be a wide-angled 1/2 in., which of course has a large back lens, 

 which the modern 3/8 in. polariser would not nearly fill. It is 

 obvious, therefore, that some other kind of polariser is required. 

 Experiments have been made with several. First, a tourmaline. 

 This is satisfactory, but the colour is a fatal objection, also 

 it is expensive. Secondly, black glass. This entails much loss 

 of light, it is a poor polariser, and not at all satisfactory. 

 Thirdly, a pile of plates, which is excellent, and practically as 

 good as a Nicol, Therefore the pile of plates has been selected 

 for this new instrument. I have already shown the value of 

 a pile of plates set in the cell in place of the concave mirror. 

 This plan answers perfectly when one is able to set, in a few 

 seconds, the mirror and source of light at the proper polarising 

 angle. It was also shown how a card cut at twice the polarising 

 angle would be of assistance. But there are some who have 

 much difficulty in putting the mirror in the proper position ; 

 when they do succeed, it is either by chance or after the expen- 

 diture of half an hour's labour. It was therefore necessary to 

 see that this new apparatus should be fool proof. A glance 

 at the figure, which is so simple that it hardly requires explana- 



