the Two Rays in Calcareous Spar, 375 



field, perfect darkness. The weak light which is still visible,* 

 is evidently derived from reflection on the lateral planes, and 

 would undoubtedly be removed if we blackened them. If, 

 while the principal sections cross one another, we insert between 

 the two rhomboids a plate of calcareous spar which has been 

 cut perpendicular to the axis, we obtain a combination entirely 

 formed of calcareous spar, which exhibits the phenomena of 

 coloured rings with the black cross as distinctly as they are 

 obtained by any other method. If we now interpose a plate of 

 mica, of the proper thinness, before and behind the plate of 

 calcareous spar, between it and the rhomboid, so that the axes 

 of these two little plates may cross each other at right angles, 

 and at the same time bisect the right angle between the two prin- 

 cipal sections, we then see the coloured rings without the cross 

 having the black centre, and quite similar to the Newtonian 

 rings. An interesting modification of these phenomena has been 

 described by Mr Airy, and was, at an early period, exhibited 

 to me by Professor Norrenberg. 



It may be remarked, that Brewster had previously employed 

 an analogous method to destroy one of the images in double 

 refracting crystals.f For this purpose, he cuts a prism from a 

 crystal of this nature, (as, for example, calcareous spar, arrago- 

 nite, saltpetre, carbonate of potash, Sec.) and in such a manner, 

 that its angles may be parallel to the optical axis — he leaves 

 two of the planes of the prism rough, and fixes glass plates 

 upon it by means of a suitable substance. If the substance 

 has the same power of refraction as the ordinary ray of the 

 crystalline prism, this ray goes through, since, in relation to it, 

 the planes of the prism are equally polished ; but the extraor- 

 dinary ray becomes scattered on all sides as on a rough plane, 

 and is thus in some measure interrupted. If the substance 

 has the same refractive power as the extraordinary image, the 

 reverse takes place. For example, if we fix the glass plates 

 on a prism of saltpetre, by means of copal balsam, or what is 

 still better, cassia oil, the ordinary ray, whose refraction is 1 ,511 ^ 



• By making the lateral planes rough, which may be done with a file, there 

 is scarcely any lateral reflection.— •£</»/. 



t Phil. Trans, for 1819, p. 146, and Edin. Encyclop. art. Optics, p. 600 



