Schulze's Scotophorus

Schulze's Scotophorus: earliest fleeting letter photograms (circa 1717)
Around 1717 German polymath Johann Heinrich Schulze accidentally discovered that a slurry of chalk and nitric acid into which some silver particles had been dissolved was darkened by sunlight. After experiments with threads that had created lines on the bottled substance after he placed it in direct sunlight for a while, he applied stencils of words to the bottle. The stencils produced copies of the text in dark red, almost violet characters on the surface of the otherwise whitish contents. The impressions persisted until they were erased by shaking the bottle or until overall exposure to light obliterated them. Schulze named the substance "Scotophorus", when he published his findings in 1719. He thought the discovery could be applied to detect whether metals or minerals contained any silver and hoped that further experimentation by others would lead to some other useful results. Schulze's process resembled later photogram techniques and is sometimes regarded as the very first form of photography.
De la Roche's fictional image capturing process (1760)
The early science fiction novel Giphantie (1760) by the French Tiphaigne de la Roche described something quite similar to (colour) photography, a process that fixes fleeting images formed by rays of light: "They coat a piece of canvas with this material, and place it in front of the object to capture. The first effect of this cloth is similar to that of a mirror, but by means of its viscous nature the prepared canvas, as is not the case with the mirror, retains a facsimile of the image. The mirror represents images faithfully, but retains none; our canvas reflects them no less faithfully, but retains them all. This impression of the image is instantaneous. The canvas is then removed and deposited in a dark place. An hour later the impression is dry, and you have a picture the more precious in that no art can imitate its truthfulness." De la Roche thus imagined a process that made use of a special substance in combination with the qualities of a mirror, rather than the camera obscura. The hour of drying in a dark place suggests he possibly thought about the light sensitivity of the material, but he attributes the effect to its viscous nature.

Scheele's forgotten chemical fixer (1777)
In 1777, the chemist Carl Wilhelm Scheele was studying the more intrinsically light-sensitive silver chloride and determined that light darkened it by disintegrating it into microscopic dark particles of metallic silver. Of greater potential usefulness, Scheele found that ammonia dissolved the silver chloride but not the dark particles. This discovery could have been used to stabilize or "fix" a camera image captured with silver chloride, but was not picked up by the earliest photography experimenters.

Scheele also noted that red light did not have much effect on silver chloride (a feature that would later be applied to be able to see while printing black and white photographs in darkrooms).

Although Thomas Wedgwood felt inspired by Scheele's writings in general, he must have missed or forgotten these experiments: he found no method to fix the photogram and shadow images he managed to capture around 1800 (see below).

Thomas Wedgwood & Humphry Davy: Fleeting detailed photograms (1790?-1802)
Thomas Wedgwood (1771-1805) is believed to have been the first person to have thought of creating permanent pictures by capturing camera images on material coated with a light-sensitive chemical. He originally wanted to capture the images of a camera obscura, but found they were too faint to have an effect upon the silver nitrate solution that was advised to him as a light-sensitive substance. Wedgwood did manage to copy painted glass plates and captured shadows on white leather as well as on paper moistened with a silver nitrate solution. Attempts to preserve the results with their "distinct tints of brown or black, sensibly differing in intensity" failed. It is unclear when Wedgwood's experiments took place. He may have started before 1790; James Watt wrote a letter to Thomas Wedgwood's father Josiah Wedgwood to thank him "for your instructions as to the Silver Pictures, about which, when at home, I will make some experiments". This letter (now lost) is believed to have been written in 1790, 1791 or 1799. In 1802 an account by Humphry Davy detailing Wedgwood's experiments was published in an early journal of the Royal Institution with the title An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver. Davy added that the method could be used for objects that are partly opaque and partly transparent to create accurate representations of for instance "the woody fibres of leaves and the wings of insects". He also found that solar microscope images of small objects were easily captured on prepared paper. Davy, apparently unaware or forgetful about Scheele's discovery, concluded that substances should be found to get rid of (or deactivate) the unexposed particles in silver nitrate or silver chloride "to render the process as useful as it is elegant". Wedgwood may have prematurely abandoned his experiments due to his frail and failing health. He died aged 34 in 1805.

Davy seems not to have continued the experiments. Although the journal of the small, infant Royal Institution probably reached its very small group of members, the article eventually must have been read by many more people. It was reviewed by David Brewster in the Edinburgh Magazine in December 1802, appeared in chemistry textbooks as early as 1803, was translated into French, and published in German in 1811. Readers of the article may have been discouraged to find a fixer, because the highly acclaimed scientist Davy had already tried and failed. Apparently the article was not noted by Niépce or Daguerre, and by Talbot only after he had developed his own processes.

Jacques Charles: Fleeting silhouette photograms (circa 1801?)
French balloonist/professor/inventor Jacques Charles is believed to have captured fleeting negative photograms of silhouettes on light sensitive paper at the start of the 19th century, prior to Wedgwood. Charles died in 1823 without documenting the process, but purportedly demonstrated it in his lectures at the Louvre. It was not publicized until François Arago mentioned it at his introduction of the details of the Daguerreotype to the world in 1839. He later wrote that the first idea of fixing the images of the camera obscura or the solar microscope with chemical substances belonged to Charles. Later historians probably only built on Arago's information and much later the unsupported year 1780 was attached to it. Since Arago indicated the first years of the 19th century and a date prior to Wedgwood's process published in 1802, this would mean that Charles' demonstrations took place in 1800 or 1801 - assuming Arago was this accurate almost 40 years later.

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