444 Iceland Spar and the Polariscope. [Sess. 



subject is quite large enough without that. It will suffice to 

 say that a ray of common light, on entering a rhomb of spar, is 

 split up into two rays, — one the ordinary ray, the other being 

 called the extraordinary ray. Now we come to William Nicol, 

 the Scottish physicist, who was born about 1768 and died at 

 Edinburgh on 2nd September 1851. The 'Encyclopaedia 

 Britannica ' informs us that nothing is known of his early 

 history beyond the fact that, after amassing a small com- 

 petence as a popular lecturer on Natural Philosophy, he 

 settled in Edinburgh to live a very retired life in the society 

 of his apparatus alone. Besides the invention of the prism 

 known by his name, he devoted himself to the examination 

 of fluid-filled cavities in crystals, and of the microscopic 

 structure of fossil wood. His skill as a working lapidary 

 was very great, and he prepared a number of lenses of 

 garnets and other precious stones, some of which I have 

 laid on the table here, and which he preferred to 

 the achromatic objectives of that time. The following is 

 William Nicol's own instructions for making a prism of spar : 

 " Let a rhomboid of calcareous spar 1 inch long be reduced in 

 breadth and thickness to 3-lOfchs of an inch; let the obliquity 

 of its terminal planes be increased about 3°, or, in other words, 

 let the angles formed by the terminal planes and the adjoin- 

 ing obtuse lateral edges be made equal to 68°, by operating 

 on the terminal planes ; these planes may now be polished. 

 The rhomboid is then to be divided into two equal portions 

 by a plane passing through the acute lateral edges and nearly 

 touching the two obtuse solid angles. The sectional plane of 

 each of the two halves must now be made to form exactly 

 an angle of 90° with the terminal plane, and then carefully 

 polished. The two portions are now to be firmly cemented 

 together by means of Canada Balsam, so as to form a rhom- 

 boid similar to what it was before its division. Iceland spar, 

 when pure and free from flaws, is not only transparent but 

 perfectly colourless. A rhomboid of that substance, and of 

 the above construction, develops the coloured rings of crystal- 

 lised bodies with a degree of brilliancy not to be equalled by 

 a plate of tourmaline, nor perhaps by any other substance. 

 With the view of rendering the structure of the analysing rhom- 

 boid more easily understood, I have considered a piece of the 



