THE POLARISER. 



309 



FIG. 173. 



magnitude corresponds to the natural inclination. If we assume 

 c d 1, we obtain by trigonometrical calculation b c = *915. 

 Hence the Foucault prism is relatively less than 

 one-third the length of Nicol's. It may therefore 

 be constructed of smaller crystals of calc-spar, and 

 hence it is much less expensive. On the other 

 hand, it has the disadvantage of a very small aper- 

 ture, for the greatest possible deviation from the 

 vertical amounts to only about 4 against 12 in 

 that of Nicol. A prism of this kind could hardly 

 be used on the Microscope without a condenser. 



A few years ago Hartnack and Prazmowski 1 

 invented a construction, by which not only is the 

 prism shortened, but at the same time the angle e 

 of aperture is increased. The following are the 

 essential points of this construction: (1.) The sec- 

 tional face which separates the two halves is vertical to the 

 axis of the crystal. (2.) The faces of incidence and emergence are 

 ground at right angles to the axis of the Microscope. (3.) The 

 two halves are united by linseed oil or copaiba, whose indices 

 of refraction are taken as 1*485 and 1*507 respectively. (4.) The 

 angle which the sectional plane in which the two halves are in 

 contact makes with the incident and emergent surfaces is then 

 calculated for linseed oil 73 30', for copaiba 76 30', and for the 

 angle of aperture we get 35. The length of the prism is hence 

 reduced to 34, or at most 37 mm. 



Besides prisms of calc-spar, tourmaline has been much employed 

 in polarisation researches. Tourmaline, when cut in plates of 

 about 1 mm. in thickness parallel to the axis, possesses the 

 property of absorbing the ordinary image transmitting the 

 extraordinary refracted rays only. This property may be very 

 readily employed with the tourmalines of red to red-brown or 

 green colour, for the crystals are not only sufficiently transparent, 

 but they may be easily obtained so that plates of 5 mm. or more 

 in diameter can be slit off. But as these plates never give the 

 pure polarisation colours for the latter are modified by the 

 colour of the tourmaline itself they have not come into use 

 as polarisers, to our knowledge. They are, however, occasionally 

 employed as analysers. 



1 "L'Institut" (1866), pp. 28, 29. 



