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XVII. Account* of a remarkable peculiarity in the Structure of 

 Glauberite, which has one Axis of Double Refraction 

 for Violet, and two Axes for Red Light. By DAVID 

 BREWSTER, LL. D. F.R.S. LOND. & EDIN. 



K, -t, 



A '.'- HSU '. :M{iiVj ; v-> 



(Read 1th January 1829J 



AN the optical and mineralogical classification of crystals which 

 I published in the article Optics in the Edinburgh Encyclopae- 

 dia, I have arranged Glauberite among those in which I dis- 

 covered two axes of double refraction. The specimen which 

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

 measure the inclination of the lines of no polarisation, or ascer- 

 tain with any accuracy the laws of its action upon light. Mr 

 WILLIAM NICOL, whose ingenuity is already well known to this 

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

 he had skilfully prepared for showing its 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 generally 

 disappear. The cause of this irregularity immediately shews it- 

 self, when we expose the crystal to homogeneous rays. In the 

 red rays, we observe the phenomena of two distinct axes, the in- 

 clination 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, exhibiting the system of rings 

 round a single axis of double refraction. In all these cases, the 

 character of the principal axis is negative. It seems to be per- 



