180 Astronomical and Nautical Collections. 



The two lateral groups, on the contrary, depend on the 

 interference of the rays which have undergone different 

 refractions in the two pieces ; and the ordinary rays moving 

 the most rapidly in both the crystals which have been men- 

 tioned, the left hand group of fringes must be formed by the 

 combination of the extraordinary rays of the left hand plate, 

 with the ordinary rays of the right hand plate, and the re- 

 verse for the right hand group. 



We have now to consider the direction of the polarisation 

 of the pencils which interfere, in order to determine the 

 effects of the polarisation on the interference. It is natural 

 to suppose, that the polarisation must be, as in thicker cry- 

 stals, in the direction of the principal section, and in a direc- 

 tion perpendicular to it : but since this supposition is con- 

 trary to an ingenious theory of one of our most celebrated 

 natural philosophers, it is necessary to confirm it by an ex- 

 periment ; which may be done by cutting one of the edges 

 of the plates obliquely, and obtaining a prismatic sepa- 

 ration of the rays, which may then be directly shewn to be 

 polarised according to the supposition : and if this were not 

 reckoned sufficient proof, we might obtain it from the con- 

 sistency of the solution which it affords, with the first experi- 

 ment of Mr. Arago. It follows of course, that when the axes 

 are parallel, the rays of the same refraction are polarised by 

 each in the same direction, and those of each refraction are capa- 

 ble of interfering respectively in the middle. When the axes 

 were at an angle of 45° the rays of contrary descriptions were 

 capable of producing some effects on each other, as well as 

 the rays of the same description : so that there were three 

 groups of fringes. Lastly, when the axes are perpendicular 

 to each other, the rays of the same refraction are polarised 

 in directions perpendicular to each other, so that the central 

 group, which was formed by them, disappears, while the or- 

 dinary rays of the left hand plate are then polarised in the 

 same direction with the extraordinary of the right hand, 

 which causes the right hand group, produced by these rays, to 

 attain its maximum of intensity : while the left hand group 

 acquire the same magnitude from the opposite refractions. 



There is a third experiment, which still further confirmjj 



