310 POLARISATION. 



The same drawback is experienced with Herapathitc crystal*, 

 as recently recommended by Haidinger. Herapathite certainly 

 polarises the light very perfectly even in plates of ^ mm - i ]1 

 thickness ; its green colour is, however, detrimental. Moreover, 

 large plates are difficult to obtain, and are therefore expensive. 



Under these circumstances the Nicol prism is preferable as a 

 polarising apparatus. As ordinarily constructed its aperture is 

 large enough to provide extraordinary rays for a sufficiently 

 extensive field of view; whilst its utility has been considerably 

 increased by the improvements of Hartnack and Prazmowski. 



The Nicol may be mounted to drop into the stage opening from 

 above, or it may slide in beneath after the manner of a cylindrical 

 diaphragm. The diaphragm should be placed as near as possible 

 above the terminal face of the prism, and, if necessary, should 

 be replaced by a condenser to increase the convergence of the 

 rays, or it may be attached to the condenser. Achromatic con- 

 densers are as superfluous here as with illumination by ordinary 

 light; a plano-convex lens of sufficiently large diameter answers 

 the same purpose. 



As regards the position of the Nicol in relation to the source 

 of light, Mohl 1 recommends that the end face of the rhomb be 

 adjusted vertically to the axis of rotation of the mirror, in order 

 to bring into play the light partially polarised by the reflexion 

 at the surface of the mirror. In opposition to this, Valentin 2 

 states that the loss which the mirror produces by partial 

 polarisation varies with the position of the sun, because the 

 light from the sky (as Brewster has shown) is already partly 

 polarised in a plane passing through the visible expanse of the 

 sky, the sun and the eye, or in a plane which intersects the eye 

 and the expanse of sky vertical to it. From this we see that no 

 general rule can be laid down as to the most favourable position 

 of the Nicol. 



In our opinion Valentin's objection is without foundation. 

 For the loss which the light proceeding from the sky suffers 

 through the illuminating mirror, is wholly independent of that 

 which the reflected light suffers by its passage through the 

 prism. The former is determined for the observer by the position 

 of the room he is working in, and the position of the mirror 



1 PoggendoriFs " Annalen," Bd. cviii. (1859), pp. 181-183. 



2 "Die Untersuch. der Pflanzen- und Thiergewebe im polaris. Licht," p. 101. 



