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Notice concerning an Improvement in the Construction of the 

 Single Vision Prism of Calcareous Spar. By William 

 NicoL, Esq. F. R. S. E. Communicated by the Author. 



The single vision prism of calcareous spar which I invented 

 several years ago, and which is described in this Journal, 

 vol. vi. p. 83, and vol. xvi. p. 372, though well adapted for 

 examining various phenomena depending on the polarization 

 of light ; yet, on account of its length being so much greater 

 than its breadth or thickness, the field of view is less than 

 could be wished. This imperfection may be greatly dimi- 

 nished by shortening the prism without altering its thickness. 

 The shortening is accomplished by cutting off equal portions 

 from each extremity ; and when the prism is thus shortened 

 to half its former length, the field of view will be found greatly 

 enlarged. 



To construct a prism, then, on the shortened principle, two 

 rhomboids of spar must be procured, and if the shorter di- 

 agonals of their terminal planes be four-tenths of an inch, the 

 length of the sides or edges may be five-tenths of an inch. 

 One of the terminal planes of each must be inclined to the 

 obtuse lateral edge at an angle of 68°, and polished. One- 

 half of each rhomboid must then be removed by acting ob- 

 liquely on the obtuse edge, in such a mannei- that the plane 

 produced shall form an angle of 90" with the terminal plane. 

 When these planes are duly polished, they are to be united 

 by Canada balsam. The smaller extremity of each portion 

 must then be rendered opaque, which may be done by pasting 

 on it a piece of black tracing paper. 



A prism of the above dimensions will be found well adapted 

 for the general purposes of analysis, but one of a still smaller 

 size will be found equally efficacious, viz. even when the shorter 

 diagonals of the terminal planes are only three-fourths of an 

 inch. When a shortened prism is to be applied to the eye- 

 lens of a compound microscope, however, one of larger di- 

 mensions will be found to answer better. The one I have 

 found to furnish the greatest field, has the shorter diagonal 

 of the terminal planes five-tenths of an inch, and the length 



