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Herschel's Light

Herschel's Light

It stands to reason that photographs - 'sun pictures' as they were often called at first - were dependent on the amount and quality of the natural light, and on variables such as weather and atmospheric pollution. In fact there were other more minute variables - such as the colour, refraction, reflection, and heat absorption of the glass under which it was necessary to expose the photographic papers (to keep them still and flat during long exposures) (see for instance narrative to 95408). Nor was the relationship between photo-chemical effect and its apparent cause, sunlight, the straightforward transaction it seemed, as Sir John Herschel's researches demonstrated.

Morning light is better than afternoon; the spring months - the very time of Herschel's early experiments, in February to April 1839 - are more photogenic than the sunny summer months; the tropical sun takes longer to form a photograph than the sun in England. The question arose as to whether light alone, or even light as such (defined by the human eye), was the agency of the photo-chemical changes that were the basis of the newly invented process. For one thing, heat was observed to be a factor, and of course was inseparable from the effects in play when exposing a photographic paper to sunshine. But the influence of heat did not exactly coincide with that of light. Herschel's father Sir William Herschel had established that the heat generated at the red end of the spectrum was far greater than at the blue, and greater still a little beyond the red, where the human eye perceived no light or colour at all (infra-red).

Moisture too was more than merely another variable. Herschel noticed in 1839 that it was capable of acting as a photographic agent itself, bringing out or developing an image - it is one of the techniques represented in the invisible writing experiments (23385, 53873, 78062). He returned to it in 1843, inventing a process of considerable quality (in terms of the resulting detail and clarity) in which a latent image was developed merely by breathing on the paper (66017, 83690, 96216). Alternatively the latent image could be developed by placing the photograph between sheets of moist paper (his 'Vapour Book'; see 83690). The moisture in these cases, of course, still acts upon an existing chemical change, even if invisible, that has been caused initially by exposure to light.

Unlike most early photographic trials, Herschel's experiments were so carefully observed and scientific that he quickly realised there were forces at work which the term photo-, meaning light, did not accurately describe. For scientific uses he adopted the term 'actinism' and related formations such as actino-chemistry, the Greek actino- meaning ray. The most important scientific observations that accompanied Herschel's photographic work were his analyses of the effects of the spectrum on photographic chemicals. He used a device with a prism to split light into its seven colours (or nine - he found a band of colour at either end, additional to those recorded by Newton) and project them on to a piece of sensitised paper. The red slightly and the blue-violet area significantly darkened the paper, while in the yellow region (where the visible light is strongest) there was no effect, nor beyond the red; but at the other end of the spectrum for some distance beyond the violet (ultra-violet) the darkening was even greater. Unknown and invisible rays accompanying light but distinct from it were thus the chief talents behind the 'drawing' of 'sun pictures'.

Herschel was not the discoverer of actinic or ultra-violet radiation, but his experimental confirmation and analysis of it was a momentous scientific observation. Actual spectrum tests of this kind - in effect the first spectrum photographs - do not appear to survive in the Oxford collection (though Schultze perhaps mistakenly implies they do); there is no particular reason why Herschel should have bothered to fix them, if indeed they were fixable. But a series of notes about these experiments is among the small group of manuscripts in Herschel's hand, and includes tiny sketch charts or graphs with sensitivity curves of the chemical and heating effects of the spectrum. They are dated in December and March, probably 1842-43 (13106, 18517, 81984, 99569).

Against this background of serious scientific investigation, the notes about weather and light conditions (and so on) that Herschel occasionally added to his actual experimental photographs can be seen to have more profound purpose than their obvious practical one, of finding conditions that suited the photographic chemistry (or vice versa). Indeed, he noted such things so seldom that each example is obviously a very unusual type of experiment. The 'Watery gleam 6½ PM [sun symbol] low' and the 'watery setting [sun symbol] gleam' of May 25, 1843 (83690 and 96216) is a latent-image experiment, in which the sun's low power and position were being deliberately put to the test. There was 'no trace of picture till breathed on'. These are the vapour images mentioned above, certainly one of his oddest inventions; but the ability of a small coating of moisture or humid air to develop a latent image, and the ability of a low setting sun in ten minutes exposure or less to create such a latent image, are both significant scientific observations.

A peculiar cyanotype experiment of 1842 (98786, entirely blue) in part tested the opposite. Inspected and re-exposed on October 21 and then again in July 1843, the lengthy note filling the back refers twice to it having been 'over sunned', the implication perhaps being that he attributed its peculiar disappearances and re-appearances to that. See also 59306 ('over Sunned'), 27213 ('greenish glass'), and xxxxx ('hazy then cloudy'??). On March 4, 1839 he deliberately took advantage of adverse conditions to make a series of experimental photographs under a badly polluted sky, recording 'East Wind & London Smoke' (22719, 74474, 83686). A rare east wind (instead of the prevailing English south-westerly) brought the famous smog of Victorian London over Herschel's house in the cleaner rural town of Slough, about 20 miles directly west. The results 'came off very satisfactory', according to one of his annotations.

As astronomer, chemist, and pioneer photo-chemist, Herschel was recognised as one of the period's great authorities on light, or radiation as it now became necessary to call it. He said that light was 'his first love' {QUO??}, and he made investigations of the wave theory and the dispersion of light. His long article on "Light" for the Encyclopaedia Metropolitana (1827, revised 1845) is equivalent to a book in its own right, a significant and authoritative stock-taking of what was known about this increasingly key branch of physics. The Museum has the copy which Herschel himself gave to the optical manufacturer George Dollond. It helped re-establish the wave theory, gave it a sound mathematical interpretation, and was a standard work until the electro-magnetic discoveries later in the century.

And of course, not least, Herschel established the scientific basis of photography, as well as showing that photography was not merely a technique for making pictures, it was itself a fertile means of investigating the nature and effects of light. Herschel's making many practical contributions to the invention's improvement and success is only part of the story behind his photographic experiments of 1839-43, which also have their place as a classic series of experiments at the root of modern photochemistry and the science of light.

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