TALBOT4784

London
29 March 1843

Dear Sir

Many thanks for the specimens, some of which I have retained as desired, but should be glad to have a memorandum of their nature, these are N^o 844 black, & 780 blue; both negative. ¹

With respect to the name “Celænotype” ² it appears to me not sufficiently descriptive; for the positive Calotype process affords copies of Engravings equally sharp and black, & which might be mistaken for real engravings – But as your new process involves a very remarkable peculiarity viz. the change from negative to positive of the same photograph, I should wish the name given to it to be one allusive to that fact, and if you are not yet decided upon your nomenclature I would suggest that the above peculiarity might be concisely and clearly expressed by the name of Amphitype. The Greek name for “to change” [illegible deletion] is somewhat too long for convenient use (αλλασσειν) unless abbreviated into allotype; but the other is more classical.

I mentioned that we saw on the 24^th besides the comet ³ a nebulous light involving the Pleiades – I find you regard this as the true Zodiacal light: but it still remains to me very obscure why the Z. light does not appear during total eclipses if it is really produced by the Sun’s atmosphere; and therefore I think this phenomenon must have its origin in our own atmosphere.

The slow changes which take place in your photographs during several months are certainly very remarkable; do not you think they might be hastened by constantly keeping them in a warm place, or in a reservoir filled with some gas or other?

Mess^rs Johnson & Wolcott ⁴ have made a new improvement in the Daguerreotype; I saw a specimen yesterday which I think the most perfect thing of the kind I have yet seen, and leaving little to be desired in the way of further improvement, if its execution be not attended with any unusual difficulties. A newspaper called the Illustrated London News contained a good woodcut of the Comet as seen at Blackheath on the 17^th inst – quite like its appearance as I saw it on the 24^th only that it was much fainter then. This seems a very remarkable comet, its tail grows faint without growing shorter, or materially shifting its place in the heavens. Is it going away from us nearly in a straight line?

Talking of comets, in my more mathematical days, I once found out the following simple theorem –

If the passage of a comet thro’ both its nodes is observed, these 2 observations suffice to determine the orbit rigorously, & by a very simple geometrical construction. Except the Inclination which remains indeterminate. The last circumstance renders this theorem a very peculiar one. I had afterwards reason to believe that my theorem was not altogether new, but I have since lost the memoranda I made on that point. The orbit is assumed parabolic.

I remain dear Sir Yours very truly
H. F. Talbot

Your N^o 4 appears to contain the elements of an excellent photogr^c method, but to have been spoiled by an incautious use of the brush.

Sir J. Herschel Bar^t