406 



CHAPTER VI. 

 DISCOVERY OF THE LAWS OF POLARIZATION. 



IF the Extraordinary Refraction of Iceland spar 

 had appeared strange, another phenomenon was 

 soon noticed in the same substance, which appeared 

 stranger still, arid which in the sequel was found 

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

 were afterwards described under the term Polar- 

 ization. Huyghens was the discoverer of this class 

 of facts. At the end of the treatise which we have 

 already quoted, he says 1 , "Before I quit the sub- 

 ject of this crystal, I will add one other marvellous 

 phenomenon, which I have discovered since writ- 

 ing the above; for though hitherto I have not 

 been able to find out its cause, I will not, on that 

 account, omit pointing it out, that I may give oc- 

 casion to others to examine it." He then states 

 the phenomena; which are, that when two rhom- 

 bohedrons 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 

 extraordinary ray is only extraordinarily refracted 

 by the second crystal, neither ray being doubly 

 refracted. The same is still the case, if the two 

 1 Tr. Opt. p. 252. 



