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



companied by a number of extremely minute obscurely coloured 

 fringes, where it terminates on the other. If the rhomboid re- 

 volve beyond the fringes, the ordinary image will disappear, 

 and the extraordinary image come into view. The latter, how- 

 ever, from the great obliquity of the incident light, occupies a 

 smaller range, and is less distinct than the other. The ordinary 

 ray passmg out of the rhomboid in a direction parallel to its la- 

 teral edges, is therefore the best adapted for analytical purposes ; 

 and as calcareous-spar, when pure, and free from flaws, is not 

 only transparent, but perfectly colourless, a rhomboid of that 

 substance, of the above construction, developes the coloured 

 rings of crystallized bodies with a degree of brilliancy not to be 

 equalled by a plate of tourmaline, or perhaps by any other sub- 

 stance. 



With the view of rendering the structure of the analyzing 

 rhomboid more easily understood, I have supposed a piece of 

 the spar to be divided into two equal portions. Such a division, 

 however, would be a difficult task ; but if two similar pieces of 

 spar be taken, it will be found a very easy matter to remove 

 one-half of each of them, either by grinding, or by the action 

 of a file. The pieces of spar should not be much less than an 

 inch long, and they need not be longer than 1.4 inch. If the 

 latter dimension be adopted, the breadth and thickness will re- 

 quire to be about .48 of an inch. 



In cementing the two pieces together, it will be proper to let 

 the pointed end of the one project a little over the terminal plane 

 of the other. By so doing, a more firm contact is obtained at 

 the edges ; and when the cement is sufficiently indurated, the 

 whole of the projecting parts may easily be removed, according 

 to their cleavages. The lateral planes should be left quite 

 rough, to prevent the reflection of extraneous light. 



Theory of Compression applied to discover the Internal Con- 

 stitution of our Earth. By Professor Leslie. Commu- 

 nicated by the Author. 



X HE subsequent note is contained in the new edition of my 

 Elements of Natural Philosophy^ about to be published. It 

 contains a rapid sketch of some curious speculations relative to 



