USE OF DOUBLE-IMAGE PRISM. 247 



may be exhibited as large, and with as much brilliancy, as 

 in Norremberg's polariscope. 



These coloured rings form a curious study ; when, for 

 instance, a section of Iceland spar (a negative uniaxial 

 crystal) is cut from I to 20 millimetres thick in a plane 

 perpendicular to the axis, and observed with polarised light, 

 the following curious appearances are presented. First, 

 when the axes of both polariser and analyser are perpen- 

 dicular, there is shown a beautiful series of coloured rings 

 traversed by a black cross ; and secondly, when the axes 

 are parallel, the coloured rings possess complementary tints 

 to those shown at first, while a white cross has taken the 

 place of the black one. 



In biaxial crystals the system of rings is more complicated, 

 and this series may be well studied in a plate of nitrate of 

 potash, cut perpendicularly to its axis. 



In order to exhibit the phenomenon of double refraction 

 a " double-image prism " is used over the eye-piece, in con- 

 junction with an objective; a plate of brass, pierced with a 

 series of holes T V to \ of an inch in diameter, being laid 

 upon the stage. When the polariser is fitted into its place 

 below the stage, and the double-image prism over the eye- 

 piece, there will appear (in some positions) two images, 

 while when the prism is moved, one of the images will be 

 completely obliterated, a change taking place at every 

 quarter of a revolution. When the polariser is taken 

 away the eye-piece shows two distinct images of the 

 light circle, the ordinary and the extraordinary ray, which 

 describe a circle upon revolving the eye-piece. If one 

 of the larger apertures be now placed under the objective 

 the images will not be completely separated but will overlap. 



These are only a few of the many curious effects produced 

 by polarised light, and its study should be more fostered 

 and encouraged. 



