a 
DOUBLE REFRACTION. 199 
in the search after causes, we are often reduced to pro- 
ceed by the method of exclusion, and in this point of view, 
there is no experiment which is without use; we can- 
not multiply them too much. That universal genius, Vol- 
taire, who often took pleasure in concealing a profound 
meaning under a burlesque form, compared a theory to a 
mouse, which passes, he said, through nine holes, but is 
caught in the tenth. It is in multiplying indefinitely the 
number of these holes, or to speak in a manner less triv- 
ial, the number of tests which a theory ought to satisfy, 
that astronomy is placed in the rank which it occupies 
in the estimation of men, and that it has become the first 
of the sciences. It is in following the same route that 
we shall be able in like manner to give to other branches 
of science the character of evidence which they yet want 
in some respects. In every science of observation we 
must distinguish the facts, the laws which connect them, 
and the causes. Often the difficulties of the subject 
arrest experimenters after the first step; hardly ever do 
they allow them to pass freely to the third. The pro- 
gress which Fresnel made in the two former respects, in 
the study of double refraction, by natural consequence, 
led him to inquire on what so singular a phenomenon 
depended. And here again he obtained striking suc- 
cess. But pressed for time, I can only make known the 
most prominent of his results. 
When Huyghens published his 7raité de la Lumiére, 
there were only known two crystals possessing double 
réfraction,—carbonate of lime and quartz. At presentit 
would be far shorter to enumerate those which have not 
this property, than those which have it. Formerly, it 
was necessary that a substance should distinctly show 
double images, to allow us to assimilate it with Iceland 
