In the optical and mineralogical classification of crystals which I published in the article Optics in the Edinburgh Encyclopædia, I have arranged Glauberite among those in which I discovered 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 ascertain 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.