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Herschel's Drafts

Herschel's Drafts

In addition to the draft letter (see narrative to 34848), four pieces of scrap paper used as spacers in the ageing test (see 94276) have been found to contain writings by Sir John Herschel about his photographic experiments. They were identified by Schultze as being two different drafts for a section of Herschel's Royal Society paper of 1840. Section 114 of this major paper on photo- and photographic chemistry discusses, in a manner typical of Herschel's meticulous scrutiny of even the tiniest variables and details in his experiments, the degree to which the photo-chemistry and the formation of the photographic image may be affected by things like the glass under which the experimental papers were exposed, the refractive and reflective properties of the glass, the creation and retention of heat during exposure, and similar factors.

In some senses the discussion is fussy. Though at the minute level at which such experiments were being conducted (that is, with small quantities of chemical and very slight changes) it was wise to be alert to even the slightest variable in cause and effect. One conclusion, for instance, as Herschel says, was that heat was indeed an active factor. It was Herschel's alertness to such precise observations that gave rise to the most considerable scientific observation to arise from his photographic experiments - though not mentioned in this draft (see narrative to 13106) - which was that the invisible part of the spectrum of light beyond the violet (ultra-violet) had a stronger photo-chemical effect and consequently a greater role in the creation of photographs than the visible spectrum.

The two drafts on the four scraps of paper are transcribed below. They were written in 1839. As well as differing from each other, the final article as published has further revisions, both of expression and in the way Herschel's conclusions are argued.

[First draft]
'It was not till I had satisfied myself that this effect which is not produced, or very little in cloudy days, and which is the more marked the more strong the sunshine, did not originate in any of the more obvious causes such as the return of reflected light from the surfaces of the glass after reflection at the paper, or from the front surfaces after reflection at the back, from heat developed and kept, from the retention of moisture by the glass causing an increased susceptibility of the paper - nor even from heat developed at the point of contact and retained by the glass - although certainly heat does{??UL} exalt the susceptibility and in many trials the glass was found to be considerably warmed.'

[Second draft]
'The remarkable effect in question is not produced, or very little in cloudy days, and is the more marked the more strong the sunshine. It does not however originate in any of the more obvious causes such as the return of an additional quantity of light from the surfaces of the glass after reflection at the paper, or from the front surfaces after reflection at the back, nor yet from the retention of moisture ...{???} by the glass causing an increased susceptibility of the paper - nor even from heat developed at the point of contact and retained by the glass - although certainly heat does{??UL} exalt the susceptibility and in many trials the glass was found to be considerably warmed.{??} - Of the insufficiency of all these explanations I satisfied myself by experiments purposely instituted.'

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