POLARIZATION. 181 



(forming the ordinary polariscope) or two plates of the 

 mineral called tourmaline cut in the direction of the 

 length or axis of the crystal. 



Hitherto we have considered rays of light falling 

 upon transparent substances as simply refracted or 

 reflected according to the ordinary laws of refraction 

 or reflexion. We have now to notice some curious 

 exceptions, forming the basis of many interesting 

 phenomena, especially in connexion with the micro- 

 scope, in which these laws are more or less deviated 

 from. If we place a plate of tourmaline, cut as above 

 directed, upon or beneath the stage of the microscope, 

 the light will pass through it, appearing tinged with 

 the green or brown colour natural to the tourmaline ; 

 but on laying another slice upon the eye-piece, and 

 turning the latter round or rotating it, the light will 

 be transmitted in certain positions only, being par- 

 tially or entirely arrested in others, so that the field 

 appears black. And, on careful examination, it will 

 be noticed that the change from black to white 

 occurs at each quarter of a rotation, being twice black 

 and twice white in an entire rotation, the changes 

 occurring alternately. The same phenomena may 

 also be exhibited by substituting two Nicol's prisms 

 for the tourmalines. 



Again, if we take a natural crystal of calcareous 

 spar, and paste upon one side of it a piece of black 

 paper with a small hole in the middle, on holding the 

 crystal to the light or over a piece of white paper, 

 with the covered side next the light, two holes or two 

 images of the hole will be seen ; and if the crystal 

 without the paper be placed over some print, the print 

 will appear double. Hence the light passing through 

 the hole is twice or doubly refracted, one ray following 

 the ordinary law of refraction, while the other follows 

 a different law, being retarded and pursuing a longer 

 course ; and so the two rays are called respectively 

 the ordinary and extraordinary ray. And on viewing 



