168 



ASTRONOMICAL AND NAUTICAL 

 COLLECTIONS. 



i. Elementary View of ifA^ Undulatory Theory of Light, 

 By Mr. Fresnel. 



[Continued from the Number for April.] 



Of Double Refraction and Polarisation, 



When we throw a luminous pencil on one of the natural faces 

 of a rhomboid of calcarious spar^ it divides itself within the 

 crystal into two other pencils, w^hich follow different paths, 

 and then present two images of objects seen through the 

 rhomboid. This phenomenon has been distinguished by the 

 name of double refraction, with many others of the same kind 

 that are exhibited by other crystals, especially when they 

 are cut into prisms, in order to render the separation of the 

 images more sensible. 



This bifurcation of the light, however, is not the most re- 

 markable circumstance belonging to double refraction ; each 

 of the pencils, into which the incident rays are divided, is pos- 

 sessed of some singular properties which make a distinction 

 between its sides. In order to describe tlie phenomena in 

 question with precision, it is necessary to employ, and to 

 explain, some particular expressions. 



In such crystals, as exhibit the laws of double refraction 

 in their simplest form, there is always a certain direction, 

 about which every thing occurs in a similar manner on all 

 sides ; and this direction is called the axis of the crystal. It 

 must not be considered as a single line : for there may be as 

 many axes as there may be lines parallel to each other, and 

 yet crystals of this kind are denominated crystals with a single 

 axis, if, in all other respects, the optical phenomena are the 

 same in all directions round it : so that the word is merely 

 synonymous with a fixed direction. It must be supposed 

 that the direction of the axis depends on the crystalline 

 arrangement of the particles of the medium, and that it must 

 hold, with respect to the faces, or their lines of crystallize^^ 



