176 To make Doubly Refracting Prism [March, 



remains uncorrected. Even when the refracting angle of the 

 prism is small, this uncorrected colour is very perceptible ; but 

 when the refracting angle is great, which it is often required to 

 be when we wish to see at one view the systems of rings pro- 

 duced by crystallized bodies, the spectrum of the uncorrected 

 pencil is extremely large, and alters completely the nature of the 

 light which we are investigating. This is the form of the achro- 

 matic prism which I at first employed, and which has been 

 used by M. Biot,* and, I believe, also by Malus. 



In order to construct a prism in which the dispersions of both 

 the images shall be simultaneously cor- 

 rected, let us suppose ABC to be 

 the prism of spar, and let two prisms of 

 glass, Be^D, C e d, be constructed, 

 so that the first is capable of correcting 

 the colour of the most refracted image, 

 while the second, C e d, is capable of 

 correcting the colour of the least re- 

 fracted image. These prisms must be 

 cut in such a manner that their joining 

 surface, d e, bisects the angle, m r u, 

 formed by the two pencils when the 

 ray R r is incident in the desired 

 direction. If these two prisms are made 

 of the same kind of glass, C e d will generally have a less 

 refracting angle than BciD; and the surfaces C d, D d, will 

 form at d an angle less than 180°. But if it is thought desir- 

 able to have C d D a straight line, it may be accomplished 

 by selecting crown and flint glass of the proper refractive and 

 dispersive powers, and making B e d T> of flint glass, and C ed 

 of crown glass. The surface C d D should be covered with 

 an opaque plate, EFGH, as shown 

 in the annexed figure, having an 

 aperture, o p, a little larger than the 

 pupil, and having the line of junction, 

 d, of the prisms for one of its dia- 

 meters. 



If calcareous spar were capable of receiving a perfect polish, 

 the achromatic prism would now be complete ; but as it appears 

 to be impossible to give its a&rface a very fine lustre, we must 

 endeavour to produce, by other means, an equivalent effect. 

 W hen the surfaces of the calcareous spar are as highly polished 

 as possible, the surfaces Be, C e, of the two prisms must be 

 cemented to the surface C B by a transparent cement, whose 

 refractive power is a mean between the refractive powers by 

 which the pencils r m, r n, are influenced ; and by means of the 

 same cement, plates of well-polished and well-annealed glass 



Hi 



7 



J 

 ■? 



* See Bict'^ Traite de Physique, torn. 



268. 



