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 Traite de la Lumiere, 

 there were only known two crystals possessing double 

 refraction, carbonate of lime and quartz. At present it 

 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 



