Allerly by Melrose
Jany 6th 1837
My Dear Sir,
You must have thought me very inattentive in not having sooner answered your last letter. It has, however, till now, been out of my power. Ever since I left your hospitable roof <1> I have been a traveller, and tho’ I could have written you, yet I had not the means of answering the Question contained in your letter. <2>
I have only now got my Books and papers from the Highlands <3> so as to be able to refer to them on the subject of Dichroitic Crystals.
I find that I have given a long list of them in the Art. Optics in the Edinburgh Encyclopædia, <4> and I think it is likely that I may have added to the list in the article on Polarisation in the Library of Useful Knowledge. <5>–
If we consider Dichroism as applicable to those crystals in which the two pencils have a different intensity without a difference of colour, I am convinced that there is not a Mineral of which Dichroitic specimens may not be found.–
I conceive that the Dichroism is the result of imperfect crystallisation,– the crystal being sometimes deposited from an imperfect solution, & sometimes from a solution which deposits also foreign matter (which may be opaque or transparent, coloured or colourless, crystallised or uncrystallised. <6> The imperfect crystallisation may arise also from the presence of an isomorphous body which does not wholly fulfil the optical functions of the original molecules which it replaces; or the deposition of the crystal may be disturbed by vibrating movements, & by changes of temperature & electrical conditions. In all these cases the particles of the Crystal do not take their natural places, so that the transmitted rays experience reflections and absorptions to which they wd not be exposed in the perfectly formed body.
I have shewn that dichroism, or an inequality in the intensity of condition of the two pencils may be produced in all doubly refracting Crystals by altering their superficial state and making the light pass thro’ a medium whose Index of Refrac[tion]<7> approaches more to that of one of the penc[ils] than the other (Art. Optics p. 600 Phil. Trans. 1819 p. 146 <8>). Now this process may be the very one which produces the dichroism in Crystals containing foreign or Isomorphous matter. If the Index (or Indices) of refraction of this matter is nearer that of one pencil than the other, dichroism must be produced. If the foreign matter has one colour and the crystal another, – coloured dichroism will be produced. If the foreign matter is yellow, and the crystal blue we shall have the dichroism of acetate of copper which consists in there being more or less yellow in the two Images. I think I shall be able to shew that this hypothesis explains all the phenomena as related to the optical axes. As I can now send you Copies of almost all my Papers, I will thank you to let me know if I may send them to Sackville Street <9> in a Booksellers parcel. I beg you will offer my most respectful Compts to the Ladies of your Household and to Mr Fielding <10> whom I long to see again.
I am My Dear Sir Ever Most Faithfully yrs
At Allerly by Melrose
H.F. Talbot Esqr
1. Brewster had stayed at Lacock just before the meeting of the British Association for the Advancement of Science at Bristol, which began on 22 August 1836. See Doc. No: 03339. A plan to spend a day there after the meeting had to be abandoned. [See Doc. No: 03363].
2. Not traced.
3. Brewster had spent much time at Belleville, near Kingussie, the home of his late father-in-law.
4. D. Brewster, ed., The Edinburgh Encyclopædia (Edinburgh: Blackwood etc., 1813–1830).
5. D. Brewster, ‘Double refraction and polarisation of light’, Library of Useful Knowledge (London: Baldwin and Cradock), 1829.
6. No closing bracket.
7. Text torn away under seal.
8. D. Brewster, ‘On the action of crystallized surfaces upon Light’, Philosophical Transactions of the Royal Society of London, 1819, pp. 145–160.
9. 31 Sackville Street, London residence of the Feildings, often used as a London base by WHFT.
10. Rear Admiral Charles Feilding (1780–1837), Royal Navy; WHFT’s step-father.