CHAPTER X. 



THE POLARISCOPE. 



WHEN a ray of light is made to pass in a certain direction 

 through a crystal of Iceland spar, it becomes split up into 

 two portions of equal intensity, an ordinary ray and an 

 extraordinary ray respectively. If a ray, having undergone 

 ordinary refraction through a crystal of Iceland spar, be 

 allowed to pass through a second crystal, it is split up in 

 the same manner ; but the ordinary and extraordinary rays 

 will be of different degrees of intensity. When the second 

 crystal is rotated until the two are in similar or opposite 

 positions, then the ordinary ray appears at its greatest inten- 

 sity, while the extraordinary ray disappears. If the second 

 crystal is further rotated the extraordinary ray reappears 

 and increases in intensity as the rotation proceeds, while 

 the ordinary ray diminishes in intensity until the principal 

 planes of the crystals are at right angles to each other, 

 when the ordinary ray vanishes and the extraordinary ray 

 appears at its greatest intensity. 



These and similar phenomena are called polarisation, 

 and two prisms are required to render it patent the 

 first for polarising the light, called the " polariser," or the 

 <' polarising prism ;" and the second, necessary for exhibit- 

 ing the fact that the light has undergone modification, 

 called the " analysing prism," or simply the " analyser." 



There are several methods of producing the polarisation 

 of light, but the one usually employed for microscopical 

 purposes is by the aid of the Nicol or single image prism, 

 which yields a beam perfectly colourless, polarising com- 



