Dr Brewster 07i a peculiarity in Glauberite. 325 



Art. XX. — Account of a remarkable peculiarity in the Struc- 

 ture of Glauberite,* which has One Axis of Double Refrac- 

 tion fur Violet, and Two Axes for Red Light. By David 

 Brewster, LL. D. F. R. S. L. & E. 



In the optical and mineralogical classification of crystals which 

 I published in the article Optics in the Edinburgh Encyclo- 

 pcedia, I have arranged Glauberite among those which have 

 two axes of double refraction. The fragment which I used 

 however, was so small and imperfect, that I could not measure 

 the inclination of the resultant axes, or ascertain with any ac- 

 curacy its action upon light. Mr William Nicol, Lecturer on 

 Natural Philosophy, &c. and whose ingenuity is already well 

 known, put into my hands two specimens of Glauberite, which 

 he had skilfully prepared for showing its double system of 

 polarised rings ; and, by the use of these, I have been enabled 

 to detect a very remarkable property in this mineral. 



When examined by common polarised light, the tints of its 

 rings are exceedingly anomalous, and we seek in vain for the 

 two poles where the double refraction and polarisation gene- 

 rally disappear. The cause of this irregularity immediately 

 shows itself, when we expose the crystal to homogeneous rays. 

 In the Red rays, we observe the phenomena of two distinct 

 axes, the inclination of the resultant axes being about 5°. 

 This inclination gradually diminishes in the orange, yellow, 

 and green rays, and in the violet the two poles coincide, ex- 

 hibiting the system of rings round a single axis of double re- 

 fraction. In all these cases, the character of the principal axis 

 is negative. 



When Mr Herschel discovered the very remarkable property 

 in a specimen of Apophyllile, in virtue of which it exercised a ne- 

 gative influence over the red rays, a positive influence over the 

 blue rays, and no influence at all over the yellow ones, I showed 

 in a paper read before this Society, and printed in their Transac- 

 twns,\ that these apparently irreconcileable actions, related, as 



• Abridged from the original paper read to the Royal Society of Edin- 

 burgh, Januaiy 9th, 1828, and which will appear in the Transactions, 

 vol. xi. part ii. now in the press. — Ed« 



t Edinburgh Transactions, vol. ix. p. 317. 



