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LI. Facts relating to Optical Science. No. II. Communi- 

 cated by H. F. Talbot, Esq., M.P., F.R.S.* 



7. On Mr. Nicol's Polarizing Eye-piece. 

 IN tlie 20th volume of Jameson's Edinburgh Philosophical 

 *■ Journal (p. 83.), there is a paper by W. Nicol, Esq., en- 

 titled, " On a method of so far increasing the divergency of 

 the two rays in calcareous spar that only one image may be 

 seen at a time." The admirable contrivance by which this is 

 effected does not appear to have become generally known, al- 

 though published so far back as 1828. My own attention was 

 first drawn to the subject from seeing a translation of Mr. 

 Nicol's paper in a German journal, [Poggendorff's Annalen, 

 1833, No. 9. page 182.) the editor of which says, that although 

 upon reading the original paper he did not anticipate any 

 good result from the arrangement proposed, jet that upon 

 making the trial he found that nothing could answer more 

 perfectly than it did. He then goes on to say, that as Mr. 

 Nicol had not attempted to explain the operation of the in- 

 strument, he would endeavour to do so, in which, however, 

 1 cannot say that I think he has been entirely successful. 



Having read this testimony to the merits of the instrument, 

 I was desirous of making trial of it, and I caused one to be 

 constructed according to the directions given by the inventor. 

 I found the performance of it fully justified the praises of the 

 writer in the foreign journal. It may be described in a few 

 words by saying that it possesses the powerful polarizing 

 energy of the tourmaline united with perfect whiteness and 

 transparency, and that thus it enables us to perceive the most 

 delicate tints of the rings of crystals, which the green or 

 brown colour of the tourmaline always injures and often de- 

 stroys. But as, in my opinion, neither the inventor nor his 

 German commentator has taken a right view of the causes 

 which render this instrument so excellent, I will endeavour to 

 do so. 



To commence by; a brief description of it. 



It is formed by taking a rhomb of calcareous spar, about one 

 inch in length, and one third of an inch in breadth and thick- 

 lies- : dimensions which may easily be given to it by splitting- 

 it according to its natural cleavage planes. It will have the 

 form of a rhombic parallelopiped, with considerable obliquity 

 in its planes. It must then be cut into two equal parts along 

 the diagonal of one of its longer faces, and the two halves 

 must then be again cemented together with Canada balsam, in 



* No. I. of these Pacta will be found in onr Number for February, p. 112. 

 — Edit. 



Third Series. Vol. 4. No. 22. April 1831. 2 V 



