Slough
June 24/39 <1>
My dear Sir
When I wrote to you from Paris I was just warm from the impression of Daguerre’s beautiful pictures. <2> After reflexion I feel no way disposed to abate in my admiration. However that has not prevented my wishing that the processes which have paper for their field of display should be perfected, as I do not see how else the multiplication of copies can take place, a branch of the photographic art which Daguerre’s processes do not by his own account admit of. – The A day or 2 before I left England I believe I wrote to you stating that I had got a new sensitive paper. <3> The basis of this is Lead. I mean that a ground wash of a salt of lead is applied before the muriatic of salt. Acetate of lead is what I have used and the result is a very remarkable degree of sensitiveness combined with a sharpness & richness of effect I have not before met with. In sensitiveness I think it equals your Bromuretted paper – at least that specimen you sent me
July 6. The above is the commencement of a letter I was about to address to you some time ago – But something interrupted it and since then having had the pleasure of meeting you it became unnecessary –. I received safely per mail the very pretty specimens you sent me & pray accept my best thanks for them.
I attempted the experiment of the glass cylinder to detect the fixed lines if any in the Chemical Spectrum. The Expt however is too refined for my present possibilities in the way of extempore apparatus & I commend it to your more efficient nursing as one sure to produce some capital result only I begin to doubt whether the Cylinder be so good a mode as others for diminishing the angular breadth of the luminous line.
Failing, and bungling this, I tried the naked Spectrum, and here to my surprise & delight I find that there are facts in abundance to reward research. – My prism was a beautiful one given me by Fraunhofer <4> of his heavy Flint. In this the spectrum formed by this I observe as Follows –
1st The Chemical action extends enormously beyond all visible light – The end of Visible Violet as seen through a Cobalt glass, about bisects the Chemical Spectrum.
2d– The maximum of effect is so far from lying in the Violet rays that it may be rather placed on the confines of the blue & green – about Fraunhofer’s ray F or the strongly marked line between F & G about as far from F that way as b is the other.
3d The pure red up to the line C is without action
4th But here comes the most remarkable part of the affair. The portion between C and D or rather the less refrangible half of that portion, though its action is feeble, yet differs from all the rest by Communicating a red-brown colour to the sensitive paper. – and this tint is the more decided the more completely the yellow & green rays are excluded.
Here the
5. When I say differs from all the rest, I speak rather too generally. The same ruddy tendency comes on in the portion beyond the visible violet, but in a much less decided way & only to be perceived by contrast with the very black tint produced by the blue & green. It is very curious by the way that the most refrangible rays have again a decidedly ruddy colour.
6. So far from a maximum, there is actually a minimum of effect in the Visible Violet rays about where Fraunhofer’s ray H is situated or rather about ⅓ of the way towards G.–
Here then we have your curious remark of the photographic impressions from red coloured pictures & parts of pictures being themselves analogously coloured – traced up to the action of certain definite rays in the spectrum. – Should the Sun continue good and if I could only get time to use it other than in the fidgety hurried way which [illegible deletion] I propose to try the effect of finer rays, collected in larger quantities in focus of lenses especially further towards the red end so as to make quantity compensate for feebleness of action, when it may be fairly surmised that much more decided shades of hues of red colour will be produced.– And who knows but that the black tint produced by the blue-green may not be a mixed tint of red, blue, & yellow which may possibly be insulated by analysing the incident light not by the prism, but by coloured media, and prism combined.*
I find that my leaden paper is subject to the great inconvenience of turning not only yellow but dark brown (when rendered very sensitive) by keeping even a few days. – And as I have now got the right process & proportions for your salted paper, whose sensibility is after all quite sufficient for every purpose – I prefer the latter as the trouble of preparation being the same in both, viz: 3 washes
I enclose a specimen of a retransfer from an engraving, fixed. <5> The original has a somewhat misty air especially in the distant column. The Lady’s face & neck are strongly shaded by her veil. The photograph as you will perceive has very much the air of a copperplate engraving. –
Believe me dear Sir Yours very truly
JFW Herschel
*If any coloured medium would be found which should give a pure blue, or yellow, I should by analogy expect to have a predominance of blue or yellow in the darkness produced by such a chemical ray transmitted thro such a medm. Such media however do not exist or have not been found. But by using the prism first to separate all but the pure prismatic tint of given refrangibility & then re-analysing this by media I conceive it possible to obtain rays totally exempt from any colour but that wanted the elementary one wanted. You will at once see what a fine train of Expts this opens.
But I confess I should n
Coloured flames ought to be tried & the Electric lights of metals which Wheatstone <6> has shewn to be definite & characteristic
H.F. Talbot Esqr
31 Sackville Street <7>Piccadilly
London
Notes:
1. In the sequence of their correspondence, this has been classified as belonging to 6 July, the date on which Herschel finished and dispatched the letter. Herschel recorded in his diary for 6 July that he ‘wrote to Talbot’ [HRHRC W0022].
2. Herschel wrote his impressions on first viewing the daguerreotype in Doc. No: 03875.
3. He named it ‘plumbozoic’ paper, no. 1038.
4. Joseph von Fraunhofer (1787–1826), optician, Munich.
5. No enclosure.
6. Sir Charles Wheatstone (1802–1875), scientist.
7. 31 Sackville Street, London residence of the Feildings, often used as a London base by WHFT.