200 FRESNEL. 
spar. Whenever the separation of the two rays was so 
small as to escape detection by the eye, the observer 
remained in doubt and did not venture to pronounce it 
doubly refractive. Now, however, by the aid of a 
method which a member of the Academy has _ pointed 
out,* the existence of double refraction manifests itself 
by characteristics quite independent of the separation of 
the two images. No substance, however thin it may be, 
possessed of this property, can escape this new mode of 
examination. But, if it were certain that double refrac- 
tion could not exist without our perceiving the very 
manifest phenomena on which this method is founded, it 
would not appear equally incontestable that it ought 
necessarily to accompany them; and a doubt in regard 
to this might seem the more natural since the author of 
this method has himself found certain plates of glass 
which, without separating the images in a perceptible 
degree, yet give birth to all the phenomena in question : 
—since a distinguished philosopher of Berlin, M. See- 
beck, afterwards proved that all glass rapidly cooled 
enjoyed the same property ;—and since, lastly, a very 
able experimenter of Edinburgh produced the same 
* The author here alludes to his own discovery of the polarized 
colours, made also quite independently by Brewster about the same 
time. These tints are now familiar to most persons by means of the 
little instrument called the polariscope. By placing a plate of sele- 
nite, mica, &c., far too thin to exhibit any separation of images, in 
polarized light, and viewing it through an analyzer, these brilliant 
tints convey distinct evidence of the existence of that property, since 
they are shown theoretically to depend solely upon its existence, how- 
ever insensibly small its amount may be. It therefore seems impor- 
tant for the verification-of theory, to show independently its existence 
in any substances which exhibit the tints. Glass ordinarily possesses 
no such power; but plates of wnannealed glass exhibit the tints. 
Hence the importance of the experiments mentioned to show its 
existence directly.— Translator. 
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