DOUBLE REFRACTION. 201 



phenomena by compressing pieces of glass with groat 

 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- 

 cules 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 



9* 



