LAWS OF POLARIZATION. 407 



crystals have their principal planes parallel, though 

 they themselves are not parallel. But if the prin- 

 cipal plane of the second crystal be perpendicular 

 to that of the first, the reverse of what has been 

 described takes place ; the ordinarily refracted ray 

 of the first crystal suffers, at the second, extraor- 

 dinary refraction only, and the extraordinary ray 

 of the first suffers ordinary refraction only at the 

 second. Thus, in each of these positions, the double 

 refraction of each ray at the second crystal is re- 

 duced to single refraction, though in a different 

 manner in .the two cases. But in any other posi- 

 tion of the crystals, each ray, produced by the first, 

 is doubly refracted by the second, so as to produce 

 four rays. 



A step in the right conception of these pheno- 

 mena was made by Newton, in the second edition 

 of his Opticks (1717). He represented them as 

 resulting from this; that the rays of light have / 

 "sides," and that they undergo the ordinary or 

 extraordinary refraction, according as these sides 

 are parallel to the principal plane of the crystal, 

 or at right angles to it (Query 26). In this way, 

 it is clear, that those rays which, in the first 

 crystal, had been selected for extraordinary refrac- 

 tion, because their sides were perpendicular to the 

 principal plane, would all suffer extraordinary re- 

 fraction at the second crystal for the same reason, 

 if its principal plane were parallel to that of the 

 first; and would all suffer ordinary retraction, if 



