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Herschel's Letter

Herschel's Letter

Sir John Herschel's use of scrap paper as dividers between some of the specimens of treated papers set aside for an ageing test (packet 15) has by chance preserved, torn carefully into four pieces, the first page of an early letter he wrote about his photographic experiments. It appears to have been written only a few days after the experiments commenced. Although undated, it probably belongs to February 2, 1839, as it refers to Talbot having visited the previous day - he is known to have done so on February 1. The intended recipient is not known either (it opens 'My Dear Sir'), but the fact that it launches without preamble into a report of the photographic experiments implies that it may have been addressed to an officer of the Royal Society, as Herschel kept the society informed of his photographic research from the outset. It would thus either be S. H. Christie, one of the secretaries (see 69245), or the assistant secretary J. D. Roberton. Eventually three substantial papers were published in the Philosophical Transactions of the Royal Society between 1840 and 1843. The other thing we cannot know is whether the letter was sent. What survives as scrap paper must be either a draft or an abandoned version; perhaps it was rewritten to incorporate some new development.

The surviving page ends just as he is about to mention hypo, his fixative, which was the chief discovery of the early phase of his experiments. But incomplete as it is, the letter provides a snapshot of Herschel's state of thinking on the subject at this crucial early time, exactly a week after Talbot first announced the invention and wrote to tell Herschel about it. Herschel says 'I have I believe now accomp[lishe]d the whole problem' - which sounds surprising. Yet although his research was to continue, on and off, until 1844, the statement was probably justified in terms of his initial agenda. For one of the things the letter particularly well illustrates is the clear grasp that Herschel had, from the outset, of the fundamental technical problems that photographic research must confront. He had rapidly worked out for himself, before Talbot's visit, how 'photogenic drawing' was done, and was now addressing these three problems, which he lists, that would be crucial to its viability and its future.

The fixing problem, which he mentions third in the letter, he had immediately solved by means of hyposulphite of soda (sodium thiosulphate), though perfecting its use, and convincing Talbot of it, took some more months. The sensitivity problem, which appears first on Herschel's list, was equally fundamental, especially as the original photogenic drawing process was so slow. At the time of the letter he has looked only at different silver salts. But it inspired him to commence a wider exploration of light-sensitive materials, which continued over the next several years and led to his invention of several alternative, non-silver processes, such as the cyanotype and chrysotype. From a practical point of view however the question of sensitivity was overtaken by Talbot's breakthrough in 1840, the calotype, using the latent image.

The other problem Herschel mentions - that the image was tonally reversed - seems less of a scientific challenge in retrospect. We are accustomed to the 'negative' image, and the solution seems very obvious to us; but at the time it was a new phenomenon, introduced by photography. When Herschel first made a camera image he recorded as if with surprise that the 'image was formed in White [underlined] on a Sepia-col[oure]d ground'. As late as the 1850s Robert Hunt still considered the reversal of tones a sufficiently unfamiliar thing that he included illustrations of what was meant in his textbook of photography.

It seems likely that the solution Herschel had in mind in his letter, that he says 'will require some more trials', was not a chemical but a procedural one. He did become aware quite soon that positive images could be obtained directly by a darkening and bleaching process. The early trials represented by the Oxford collection however show an emphasis on perfecting the procedure that at first he called 're-reversal' or 're-transfer' - accepting the negative, and making a second photogenic drawing from it to restore positive tones (a print, as it came to be called). The chemistry is the same, but it requires the negative to have excellent clarity and contrast, and to be translucent. This of course was the basis of most subsequent photographic practice.

The papers that form the letter bear various stains, among which, on the bottom right portion (75495), is Herschel's thumb or finger print. Photographic chemicals were of course notorious for staining the fingers. For other interesting pieces of scrap paper see narrative to 95408. The following is a transcription of the letter.

'My Dear Sir |
Mr Talbot came here yesterday & showed | me his Camera impressions. I had some also | prepared for him. I have I believe now accompd | the whole problem |
1. To find a substance greatly more susceptible | than Chlor Silver - I find that |
1. Carbonate 2 Acetate 3 Nitrate | are far more so on paper - but Carb. most |
I have not tried Benzoate but expect that to be | better still. The Carb. however does all that is re | quired. | [= the sensitivity problem]
2d. To make the drawing dark on a white ground | This I have no longer any doubt of. but to | perfect it will require some more trials | [= the negative/positive problem]
3. To render the drawing unsusceptible of | further altern. by light. This I do by' [= the fixing problem]

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