Se i 
DOUBLE REFRACTION. ri 
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 crystal 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 period before we find, I 
do not say any fresh discoveries, but even any more 
researches 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 experi- 
menters 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 
