DOUBLE REFRACTION. 221 



that the rays are polarized in the same plane. There is, 

 consequently, no need to add that two rays polarized at 

 right angles to each other must have their similar poles 

 in two directions perpendicular the one to the other. 



The two rays, the ordinary and the extraordinary for 

 example, given by any ci-ystal are always polarized at 

 right angles to each other. 



All that I have just said of polarization of light was 

 recognized by Huyghens and Newton before the end of 

 the 17th century; and never, certainly, had a more curi- 

 ous subject for research been offered to the meditations 

 of experimenters. Nevertheless, we must pass over an 

 interval of a century after that pei-iod before we find, I 

 do not say any fresh discoveries, but even any more 

 reseai'ches for the object of carrying out this branch of 

 optics. 



The history of all sciences presents a multitude of sin- 

 gular incidents of a similar kind. In the progress of 

 each science there occur periodically certain epochs when, 

 after great efforts, men usually suppose themselves to 

 have arrived at a limit in their advance. Then expei'i- 

 raenters are in general timid ; they fancy themselves 

 chargeable with a want of modesty, with a sort of profa- 

 nation, if they dare to lay an indiscreet hand on the bar- 

 riers which their illustrious predecessors have erected ; 

 and thus they generally content themselves with perfect- 

 ing the numerical elements, or filling up some deficien- 

 cies, bestowing on the inquiry a labour often arduous, 

 and which yet scarcely attracts any notice from the 

 world. 



In a word, the experiments of Huyghens had clearly 

 established the fact that double refraction modifies the 

 original properties of light in such a manner that, after 



