Slough.
Feb. 12/39

My dear Sir –

I shall mention no further the process of washing out ^<1> with the Hyposulphite if you disapprove of it & shall wait with patience for the revelation of your mode of fixing which must be a very chemical bijou. I read in the Athenaeum of Sat^{y <2>} your mention of Copying Engravings, but of course I have no idea of claiming a priority in that or any other application – though I had no knowledge of that or any part of the contents of your paper till that Athenaeum arrived, beyond what you had shewed me here. However, to do it beautifully is an art to be learned, & many and curious minutiæ will be to be reduced discovered and reduced into practice before either of us can arrive at that perfection which I am confident the thing is capable of. And it is very probable that in studying those processes each may hit on something useful in different lines and on comparing notes a process may arise better than either would have devised separately. – I find for instance that by well varnishing the backs of the Engravings with mastic varnish, the process is greatly shortened & that in very gloomy days it is practicable to get good results. – I have tried varnishing the face applying it to glass & then rubbing away the back as in the transfer of prints to wood, but it is very troublesome & difficult to do evenly. – &c &c.

Meanwhile it is not possible to multiply trials on such a subject without falling on results purely scientific & I am mistaken if I have not started a subject which it will be well worth running down. I mentioned in my last that I had met with “certain glass” which in place of enfeebling, exalted the deoxidising effect of sunshine (or daylight). In the first place I have now satisfied myself that the fact is a real one and I annex a specimen ^<3> of nitro-argented paper exposed partly covered (A) and partly exposed uncovered (B). You will not fail to notice, not only the much darker state of the covered part A, but the very remarkable difference of hue between the greenish-Brown A & the ruddy-brown B. – Two objections arise 1^st that it is the heat acquired by the glass which does it. Answer 1. The same effect takes place when the pap the exposure is not to sunshine but to the light of a perfectly cloudy & very gloomy day. 2^d that mere heat (up to 212º) does not materially accelerate the action. – Objecⁿ 2^d That it is moisture retained by the glass, while the exposed part dries. Answer it takes place just the same when the paper is well dried.

Now this has put me on other experiments and I think I am by them enabled to announce the following proposition viz. That analogous to Melloni’s ^<4> respecting what he calls diathermaneity but which I prefer calling Transcalescence (How do you like this change of name)

Propos. The Chemical rays form a spectrum of their own not identical with nor perfectly coincident with the spectrum of Calorific – or the spectra of Colorific Rays – but one sui generis – and the different rays of this spectrum are stopped by transparent media according to a scale of action differing in each medium, but by no means wholly dependent on the colour or other apparent properties of the medium nor on the quantity of the extreme violet-coloured rays which they it transmits. ^<5>

To this I feel disposed to add another– The deoxidising effect of Sunshine is a difference of the oxidising effect of certain rays & the deoxidising effect of others and that this is the case would seem decisively proved by my “exalting” glass above mentioned, unless that case be otherwise explained

A 3^d objection by the way in addition to the 2 enumerated is this – The greater efficacy of the glass arises from light reflected from the paper & back again by the 2 surfaces of the glass, so that more light is actually received by the paper exposed under the glass than by that uncovered. – The reply to this is that the effect in question is not produced by all glass. Among several specimens tried, of apparently the same tint, I have found not one other which produces any such effect. – Nor is such effect produced by 2 thicknesses of thin greenish window glass, though the increase of illumⁿ by reflected light ought to be nearly double. Nor is it produced by any coloured glass I have tried.

You will easily see what a world of interesting experiments these points are likely to broach, on the laws of chemical absorption of media solid, liquid & gaseous, coloured & colourless. I only regret that my present position, cut off almost from the use of my apparatus & chemical agents will sadly impede me in their prosecution –. However some are very obvious & very easy – & these must be tried first. – Something like your notion of a liquid floating a metal plate had occurred to me. – viz. that certain metals being covered with paper moistened with solⁿ of Platina pure water and having over that a paper moistened with a solⁿ of Platina mixed with lime water, might precipitate the latter, or rather might revive into black metal the white precipitate which light produces in that solution. – But I have found that mere daylight is not strong enough to determine the formation of that precipitate.

Supposing however as you say that Daguerre’s process ^<6> is to cover a metal plate with a liquid – it is clear that by laying a glass on this liquid and applying the flower, net, or even engraving if rendered very transparent as by enclosing between 2 glass plates holding (Canada Balsam?) – [illegible deletion] close upon that plate, it would cast a shadow by substituting a solar image in focus of a lens for the Camera-glass. – By the way this manipulation (the solar image) will enable us to fix the diffractive fringes – the Polarized Rings, &c &c on prepared paper – &c

But I shall weary you & so I remain Dear Sir Yours truly
JFW Herschel

PS Nitric acid diluted & hot does wash out a picture formed by my process. – Thereon I grounded one of my processes for photographs in Black on White without a double transfer. It met with practical success and I only laid it aside because of other things occurring.

Wedn. Feb. 13/39 ^<7>

PS.– My letter yesterday or rather yesternight was written after Post hour & I now open it to enclose a PS. stating one or 2 results of today which has been sunshiny.– 1^st I have discovered your secret of fixing – or if it is not yours it is equivalent.– The Ferrocyanate of Silver ^<8> is not affected by light. Therefore the further action of light on nitrated paper is prevented by a wash of Ferrocyanate of Potash.– This was I was led to try this by this reasoning.– Light to deoxidise Silver must displace the acid & ∴ in proportion as the acid is weak & or volatile it will displace it easier hence the nitrate & carbonate are easily affected.– But if the acid have a strong affinity & be not volatile the reverse. Now the Ferrocyanic acid is remarkable for its strong affinity for metals & in its combinations it will bear nearly a red heat.– So I tried it & with full success.– I do not ask you to tell me what your mode of fixing is but I think now I may venture to ask is it different from this? 2^d I was wrong in saying only that one specimen of glass increases the action. In bright Sun, most clear glasses do so! A very unexpected result indeed and which points out a marked difference between the direct rays of the ⊙ & those which have undergone some analysis in passing through Clouds.– The brighter the sun the greater the action of the glass. – I enclose specimens N^os 2 & 3 to shew to how very remarkable an extent this difference of action may be carried. 3^d I have I believe at length got over the chief difficulty in the way of the re-transfer & if this Sun will last a day or 2 I hope to have some good specimens – but the Ferrocyanate expert^s have led me astray from this point. I annex a specimen of paper half exposed, and fixed by the Ferrocyanate. The blue edge is given by a wash of weak persulphate of Iron.– I observe that very long exposure to sun imparts a pale tinge of sky blue to the prussiated paper which is probably owing to the decomposⁿ of a portion of the ferrocyanic acid itself by light.– I hope you will forgive my making this a double letter I have [illegible deletion] ^<9>

H.F. Talbot Esq^r
44 Queen Ann Street
London

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Notes:

1. See Doc. No: 03801.

2. WHFT, ‘Some Account of the Art of Photogenic Drawing, or the Process by which Natural Objects may be made to Delineate Themselves without the Aid of the Artist’s pencil’, Athenaeum, no. 589, 9 February 1839, pp.114–117.

3. This specimen not located.

4. Macedonio Melloni (1798–1854), Italian physicist.

5. This underlining may not be in Herschel’s hand, and may not be contemporary with the letter.

6. The daguerreotype.

7. On 13 February 1839, Herschel recorded in his diary that he “wrote to Fox Talbot a PS to letter written last night see supl Vol 5. Fine sunny day. At work all day with great interest & success at hte Photography & Chemical Rays… discovered Fox Talbot’s secret or one equivalent to it” [HRHRC, W0022].

8. True cyanates do not feature in photographic chemistry, being uncommon and rather unstable. In this case, references to ‘cyanates’ or ‘ferrocyanates’ should be read as meaning ‘cyanides’ or ‘ferrocyanides’. Thiocyanates, also earlier called sulphocyanides, did have some photographic uses.

9. The letter ends here possibly incomplete.