in Calcareous Spar and Beryl. 493 



had been considerably inclined to the axis of double refraction, 

 the images might be sufficiently separated to produce phaeno- 

 mena analogous to those in calcareous spar. When the spar 

 or the beryl is bounded by planes perpendicular to the axis of 

 the crystal, the rings become straight lines. 



In the uniaxal negative system of polarized rings which I 

 discovered many years ago in Beryl, I noticed an imperfect de- 

 velopment of the black cross near its centre. In the system of 

 polarized rings shown by the present specimen, the black cross 

 is still more imperfectly developed, the arms of the cross being 

 slightly curved and separated, as if there was a very feeble axis 

 of double refraction perpendicular to the real axis. This effect 

 is produced by a very interesting cause, to which I have had 

 occasion to refer at some length in another place*. When 

 the beryl was in a soft state from heat, the fluids or gases con- 

 tained in the tubular cavities had compressed the substance 

 of the beryl to a certain distance around them, and thus mo- 

 dified the double refraction due to the forces of aggregation 

 by which the crystal was formed. Different degrees of com- 

 pression have been produced by different cavities; but the tint 

 resulting from them is generally a full white of the first order, 

 and in some cases a yellow of that order. When in the pola- 

 rizing microscope the light is incident obliquely to the axis of 

 the crystal, and nearly in the plane of primitive polarization, 

 the regular tints are singularly modified and serrated by those 

 produced by compression. 



In the sapphire or asterial corundum, where we have three 

 systems of cavities, each parallel to the sides of an equilateral 

 triangle, the three systems of luminous lines give us by re- 

 flexion the # beautiful hexagonal radiations which are so much 

 admired. These systems of lines will be converted into rings 

 when the sapphire is cut by planes that are not parallel to the 

 direction of the cavities. 



St. Leonard's College, St. Andrews, 

 December 20, 1848. 



* On the Modification of the Doubly Refracting and Physical Structure 

 of Topaz by Elastic Forces emanating from minute cavities. — Edinburgh 

 Transactions, 1845, vol. xvi. p. 7, and reprinted in this Journal, vol. xx i . 

 p. 101, August 1847. 



