340 OPTICAL PROJECTION 



which have a disc If -inch diameter in 4 x 2 \ wooden frames. 

 A shade more may be sometimes covered. 



Some improvements have lately been made upon the Nicol 

 prism. Prazmowski's prism, as further improved by Messrs. 

 Steeg & Eeuter, is a rectangular parallelepiped, cut from 

 edge to edge instead of from corner to corner (thereby giving 

 a pencil of double the area) and with the section at right 

 angles to the optic axis. This utilises better the two refrac- 

 tive indices, and moreover the joint is made with linseed-oil 

 instead of balsam : the total result being a shorter prism, of 

 wider area and more angular field. It, however, requires 

 more than double the spar, and on account of the work, is 

 about three times the price of a Nicol of similar area. A 

 simpler and cheaper improved prism was devised by Professor 



S. P. Thompson, and is made by 

 taking an ordinary crystal of spar 

 _ 7 a very little longer than would 

 / suffice for a Nicol, and cutting the 



^^ ~.^ v / ends off so as to slant the reverse 



) way, at the lines A B, CD (fig. 

 187). Had the spar been made 

 into a Nicol the joint would have been along the dotted line 

 A D ; but the considerably shortened piece is now cut along 

 B c, the section being thus nearly at right angles to the optic 

 axis (shown by the arrow), as in the Prazmowski. This prism 

 is not so good as the latter, but is simple and easy to make. 



A complete apparatus should be fitted with analysers 

 showing the various methods of polarising light. A tour- 

 maline is easily mounted in a small cell which rotates in the 

 nozzle of the objective. Some tourmalines exhibit very little 

 appreciable colour upon the screen, but such are unfortunately 

 now becoming very scarce and costly. An analyser of thin 

 glass is easily constructed as in fig. 188, the plates being fixed 

 at the polarising angle in a tube whose end N fits into the 

 nozzle r An aperture, E, at the side of the tube enables the 



