My Dear Sir,
Having been for some days in Roxburghshire <1> I have only just now been able to read your letter with care.
I am quite satisfied that the optical phenomenon exhibited by Borax is not a case of Dichroism. The Chrystal is a Composite one such as certain Crystals of Sulphate of Potash which I have described in the Old Edinr Phil. Journal. vol. 1 p. 1 <2> The optical axes of the crystals AB, [illustration] and CD are transverse to each other, and the Mica does nothing but add its tints to, or subtract them from those of the Borax. If you incline the crystal of Borax, and look narrowly at the lines of composition which pass thro’ mn and op I am confident you will find the tints descend in the scale, and form coloured fringes.
The Sulphate of Potash which exhibits the same phenomenon is the biaxal kind which crystallises in Pyramids, and was mistaken by Count Bournon <3> for a new form viz the Bipyramidial Dodecahedron. If I recollect rightly three crystals are combined, and the coloured segments in place of being four as in Borax are Six. If I had been at home, I could have brought with me to Lacock Abbey prepared Specimens. I shall try however to find the Bipyramidal Crystals here.
I am My Dear Sir Ever Most Faithfully yrs
August 1st 1836
H.F. Talbot Esqr
1. At Allerly, near Melrose, Brewster’s home from 1827 until his death.
2. Jacques Louis, Comte de Bournon (1751–1825).
3. D. Brewster, ‘On a new Optical and Mineralogical structure, exhibited in certain specimens of Apophyllite and other minerals’, Edinburgh Philosophical Journal, s. 1, 1819, pp. 1–8.