LAWS OF POLARIZATION. 409 



stance, was found to take place at a certain definite 

 angle, different for each substance. It was found 

 also that in all crystals in which double refrac- 

 tion occurred, the separation of the refracted rays 

 was accompanied by polarization; the two rays, 

 the ordinary and the extraordinary, being always 

 polarized oppositely, that is, in planes at right angles 

 to each other. The term poles, used by Malus, 

 conveyed nearly the same notion as the term sides 

 which had been employed by Newton, with the 

 additional conception of a property which appeared 

 or disappeared according as the poles of the par- 

 ticles were or were not in a certain direction ; a 

 property thus resembling the polarity of magnetic 

 bodies. When a spot of polarized light is looked 

 at through a transparent crystal of Iceland spar, 

 each of the two images produced by the double 

 refraction varies in brightness as the crystal is 

 turned round. If, for the sake of example, we sup- 

 pose the crystal to be turned round in the direc- 

 tion of the points of the compass, N, E, S, W, and 

 if one image be brightest when the crystal marks 

 N and S, it will disappear when the crystal marks 

 E and W : and on the contrary, the second image 

 will vanish when the crystal marks N and S, and 

 will be brightest when the crystal marks E and W. 

 The first of these images is polarized in the plane 

 NS passing through the ray, and the second in the 

 plane EW, perpendicular to the other. And these 

 rays are oppositely polarized. It was further found 



