Mr Nicol on Constructing a Prism of Calcareous-spar. 83 



fed on very succulent food, have generally their liver and abdo- 

 minal organs affected with a species of hydatid, the Ci/sticercus 

 pisiformis ; and a similar occurrence is related by Bremser in 

 his work on the Intestinal Worms of Man. 



On a Metltod of so far increasing the Divergency of the two 

 Rays in Calcareous-spar, that only one Image may he seen 

 at a time. By William Nicol, Esq. Lecturer on Natural 

 Philosophy. Communicated by the Author. 



A HE following simple method of constructing a prism of calca- 

 reous-spar, so that only one image may be seen at a time, will, 

 perhaps, prove interesting to those who are in the habit of exa- 

 mining the optical properties of crystalhsed bodies by polarised 

 light. 



Let a rhomboid of calcareous-spar one inch long be reduced 

 in breadth and thickness to three-tenths of an inch ; let the 

 obliquity of its terminal planes be increased about three degrees ; 

 or, in other words, let the angles formed by the terminal planes, 

 and the adjoining 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 ex- 

 actly 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 rhomboid 

 similar to what it was before its division. 



If a ray of common light fall on the end of such a rhomboid in 

 a direction parallel to the lateral edges, the two rays into which 

 it is divided, in passing through the spar, will deviate so far from 

 each other, that only the ordinary image will be seen. That 

 image, too, will appear exactly in its true position, and free from 

 colour. The range of the ordinary ray will be found consider- 

 ably greater than the whole field of vision, as may easily be seen 

 by making the rhomboid revolve on an axis parallel to the longer 

 diagonal of the terminal planes. There is a tinge of blue where 

 the ordinary ray vanishes on one side, and a tinge of orange, ac- 



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