HO OPTICAL PROJECTION 



By taking out the front prism and substituting the Nicol 

 prism all these parts being made to fit, for obvious reasons 

 one half the phenomena will be suppressed, and the demon- 

 stration analysed and simplified. 



200. Tourmalines. The phenomena of tourmalines are 

 simple and convenient illustrations of the essential phenomena 

 of polarisation. Light brown or neutral tint tourmalines are 

 far the best ; green ones appear coarse and unpleasant. One 

 plate about an inch long and \ inch wide should be cemented 

 with balsam in the centre of a glass disc and mounted in one 

 of the standard wooden frames, and another about f inch 

 long and \ inch wide mounted on another loose disc, to be 

 used in the rotator (fig. 195). These different shapes and 

 sizes exhibit one crystal plate clearly over the other. The 



two plates are placed 

 together in the stage 



f th e optical front, 

 and the crystalg fo . 



FIG. 197. Tourmaline and Prism -, -, . 



cussed as an object 



on the screen : then as the front one is rotated the gradual 

 darkening will be seen, until total extinction is produced when 

 the two are crossed. 



These phenomena will be simply correlated with those 

 presented by the calcite, by leaving in the stage the rotating 

 tourmaline alone, and placing on the nozzle a single double- 

 image prism. It is better if this be not one of the Huygena 

 pair, but constructed of two pieces of spar after the manner of 

 Wollaston or Rochon, so as to give double the separation 

 (such an arrangement gives too much colour to be desirable 

 for Huygens' experiment). A plate of thin metal should be 

 placed in the stage with the tourmaline, having a round 

 aperture which just clears the rotating crystal and no more; 

 and the discs from this aperture should stand clear from one 

 another on the screen. Let the tourmalines first stand ver- 

 tically, as at A (fig. 197). Then one image will be transparent 



