[The original letter has not been traced and this is from WHFT's copybook copy, made by pressing tissue on the damp ink, and consequently is blurred in some areas. Brewster's incoming correspondence is thought to have been largely destroyed in a house fire shortly after his death.]
Sir D. Brewster
I have recovered the art of making the Circular Crystals <1> as well as formerly. The salt which I sent you (I mean the second quality sent) answers perfectly well. Yesterday I obtained from it the following beautiful phenomenon. I dissolved a portion with heat in a drop of water on a glass plate, pressed another glass plate upon it and then placed both glasses in a vice, under as strong pressure as the glass would bear, and left it all night in a room temperature 32o to crystallise. When taken out today the plates were examined in the first instance by the naked eye with polarised light, & analysed by a tourmaline. <2> The plates exhibited bands of colour like Newton's Rings over their whole surface. Transferred to the microscope the reason of this became apparent. The whole surface was dotted with perfect circular crystals of large size & each circle was nearly uniform in colour over its superficies, which colour varied in different circles. But it was seen that all the circles of the same colour lay together, thus for instance some hundred orange circles lay together, & these were succeeded by some hundred purple ones, then some hundred blue ones, then [original blurred] even by the naked eye alone actually produced the effect of coloured bands or zones. Is it not a new phenomenon? It is strikingly beautiful. All the circles appear fringed with gold which adds amazingly to the richness of the effect. It is caused by an incipient Secondary crystallisation deposited on the first as a nucleus. The reason of the coloured circles being distributed in zones of the same colour is simply the greater or less approximation of the glass plates in different places, the thickness of the circles being limited by that circumstance. – In order to examine this phenomenon at leisure, the microscope should be previously transported into a room temperature 32o, & made quite cold, because otherwise the heat of an apartment [original blurred] of a microscope stage of that temperature & of the hand while handling the glass plate, causes a decomposition of these beautiful crystalline circles in less than half an hour.
Believe me Yours ever truly
2. See Doc. No: 03329 for Talbot’s mounted hand-held tourmaline. He was using the principle of the polariscope to examine the interference colours produced when the two rays of polarised light are recombined into the same plane, at the analyser (the tourmaline in this case).