LAWS OF POLARIZATION". 73 



substance, which appeared stranger still, and which in the sequel was 

 found to be no less important. I speak of the facts which were 

 afterwards described under the term Polarization. Huyghens was 

 the discoverer of this class of facts. At the end of the treatise whicl 

 we have already quoted, he says, 1 " Before I quit the subject of this 

 crystal, I will add one other marvellous pnenomeiion, which I have 

 discovered since writing the above ; for though hitherto I have not 

 been able to find out its cause, I will not, on that account, omit point- 

 ing it out, that I may give occasion to others to examine it." He 

 then states the phenomena ; which are, that when two rhombohe- 

 drons of Iceland spar are in parallel positions, a ray doubly refracted 

 by the first, is not further divided when it falls on the second : the 

 ordinarily refracted ray is ordinarily refracted only, and the extra- 

 ordinary ray is only extraordinarily refracted by the second crystal, 

 neither ray being doubly refracted. The same is still the case, if the two 

 crystals have their principal planes parallel, though they themselves 

 are not parallel. But if the principal 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, extraordinary 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 reduced to single refraction, though in a different 

 manner in the two cases. But in any other position 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 phenomena was made by 

 Newton, in the second edition of his Opticks (17 17). He represent- 

 ed them as resulting from this ; that the rays of light have " sides," 

 and that they undergo the ordinary or extraordinary refraction, accor- 

 ding 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 extraordi- 

 nary refraction, because their sides were perpendicular to the principal 

 plane, would all suffer extraordinary refraction at the second crystal 

 for the same reason, if its principal plane were parallel to that of the 

 first ; and would all suffer ordinary refraction, if the principal plane 

 of the second crystal were perpendicular to that of the first, and con- 



1 Tr. Opt. p. 252. 



