Dear Sir,
I beg to thank you for your two last Notes <1> with the Circular Crystal Salt and the specimen between plates of Glass. I instantly succeeded by using plate Glass a quarter of an inch thick, and using the very method you mention in your second note, namely by squeezing the plates together with a hand vice with folds of leather intervening. My specimen, thus obtained, was magnified giving all the bands of crystals corresponding to the thickness of the space between the glasses.
I used this same method for obtaining the colour of Mixed plates, and have described it in p. 74 & 75 of my Paper on that subject in the Phil. Trans for 1838.<2>
In studying the tints given by soluble crystals that have a high refractive power it is absolutely necessary to adopt this method in order to bring their thickness within the limits of the three or four first orders of Colours. I have made several other specimens without ever failing, by following your directions.
But splendid as these crystals are, there is not one of them perfect. The crystallisation is entirely constrained, but what is very curious it does not commence at the Centre of the Crystal, as it does in the Lithoxanthate of Ammonia where the crystals are perfect, & will last for ever. I have been studying them twice a day since the 15th Jany when I first made them fm your salt, & marking their gradual decay. They will last I think [an]other 10 days. They have enabled me to solve all the difficulties which at first embarrassed me, and I think I can do no more on the subject.
I shd like to know if you possess even one crystal perfect in the centre. If you do it must be a singular accident.
If you send me your method of combining the salts &c I will insert it in my Paper. <3> I have just made two new Plates fm ye salt you have just sent me.
Believe me to be Ever Most Truly yrs
D Brewster
St Leonards College
Feby 2d 1848
Notes:
1. The first is untraced, the second is Doc. No: 06094.
2. D. Brewster, ‘On the colours of mixed plates’, Philosophical Transactions of the Royal Society of London 1838, pp. 73–78.
3. Perhaps D. Brewster, ‘Notice of experiments on Circular Crystals’, British Association for the Advancement of Science Report, 1849, (pt. 2), p. 6.