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DOUBLE REFRACTION. 201 
phenomena by compressing’ pieces of glass with great 
force in certain directions. To show that a piece of 
ordinary glass, thus modified by cooling or compression, 
always really separates the light into two rays,—and to 
render this separation incontestably evident, was the’ 
important problem which Fresnel proposed to himself, 
and which he resolved in his usual happy manner. 
By placing in the same line, and in a frame of iron 
carrying powerful screws ingeniously arranged, a number 
of prisms of glass, which by these screws were subjected 
to very powerful pressure, Fresnel caused a manifest 
double refraction to appear. In an optical point of view 
this assemblage of pieces of common glass became a true 
Iceland crystal; but here the separation of the images, 
and all the other properties which flow from it, resulted 
exclusively from the action of the compressing screws. 
Now this action, carefully analyzed, ought only to pro- 
duce one effect, a close approach to each other of the 
molecules of the glass in the direction of the pressure ; 
while, in the direction perpendicular to this, the mole- 
eules preserve their original distances. Can we then 
doubt, after this remarkable experiment, that an analo- 
gous arrangement of the molecules produced during the 
act of crystallization was thus the general cause of the 
double refraction in carbonate of lime, quartz, and all 
minerals of the same kind. If we consider with atten- 
tion the ingenious apparatus, by the aid of which Fres- 
nel, in thus giving an artificial double refraction to ordi- 
nary glass, has caused so great a step to be made in the 
science, we are struck with the great amount of aid 
which the spirit of invention borrows, whether from the 
knowledge of the arts, or from that manual dexterity 
which has been so well described by Franklin when he 
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