Astronomical and Nautical Collections. 1% 



affects the rays of light in the same manner as if it had one 

 axis only in an intermediate direction ; or a plate of rock 

 crystal parallel to the axis, and of very uniform thickness ; 

 this plate we cut into two pieces, and place one of them oft 

 each of the parallel slits. Supposing now the halves to be 

 60 placed, that the edges which were separated remain pa- 

 rallel to each other, the axes will also be parallel ; and in 

 this case we observe but one system of fringes in the middle 

 of the enlightened space, as before the division of the plates. 

 But if we turn one of the pieces in such a manner as to 

 destroy the parallelism of the axes, we form two other 

 groupes of fainter fringes, one on each side of the former 

 group, and completely separated from them, in the white 

 light, when the plates of either crystal are about the twenty- 

 fifth of an inch in thickness : and it is to be remarked, that 

 the number of breadths of the fringes comprehended between 

 the middle of one of these groups and that of the central 

 group is proportional to the thickness of the plates, for cry- 

 stals of the same nature, or in which the double refraction is 

 equally marked, as in rock crystal and the sulfate of lime. 

 In proportion as the angle formed by the axes increases, the 

 new groups of fringes become more and more distinct, and 

 acquire their greatest brightness when the axes of the two 

 plates are perpendicular to each other : in this case the central 

 group, which had gradually become fainter, altogether dis- 

 appears, and is succeeded by a uniform light ; so that we 

 must conclude that the rays which produced it by their inter- 

 ference, are no longer capable of acting on each other. It 

 is easy to infer, from the situation of these fringes, that they 

 were formed by the interference of the rays which had under- 

 gone the same kind of refraction in the two plates, since, 

 having passed through them with equal velocities, they must 

 have arrived at the same instant at the middle of the en- 

 lightened space, the plates being supposed to be of equal 

 thickness, and to remain perpendicular to the light ; and 

 these central fringes were consequently formed by the inter- 

 ference of the ordinary rays of the first piece with the ordi- 

 nary rays of the second ; and by that of the extraordinary 

 rays of the first, with the extraordinary rays of the second. 



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