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Herschel's Invisible Writing

Herschel's Invisible Writing

Three of the papers forming Sir John Herschel's photographic researches are experiments in invisible or secret writing (invisible ink), conducted in October 1839. Several different chemical methods appear to have been tried. In one the writing is made to disappear by the bleaching effect of corrosive sublimate, and then restored by hypo. This experiment was also conducted on several images (43358 and 98244), and was thus an extension of his serious investigation of bleaching and restoring actions in photographic chemistry. So too was a process where vapour (in the form of steam or breath) brings out the image, first noticed in 1839 and worked on again in 1843. This process is represented by several rather peculiar photographic images (66017, 83690, 96216). Herschel called the latter 'dormant pictures'.

The subject of invisible writing of itself falls into the uncertain area between photography and chemistry, manipulating the darkening and lightening characteristics of certain chemical compounds and conditions; but it is not clear whether any of the three specimens are truly photographic (that is, formed purely by the action of light on the chemical). The chemistry nevertheless was serious, and the invisible writing and the vapour phenomena are both mentioned in Herschel's 1840 Royal Society paper. They are relevant in particular to the phenomenon of the latent image and its 'development', and anticipate Talbot's discovery of this in September 1840. Again Herschel returned to latent and semi-latent image processes in 1842-43, including (as well as the vapour images) the chrysotype and amphitype (see narratives to 21518 and 65890 respectively). The latent image, of course, was to become fundamental to photography's future.

But experiments such as the invisible writing also illustrate a certain playfulness in Herschel's character. The earliest of the three, made on October 14, was evidently conducted with an audience, and perhaps entertaining them was its motivation. After two lines of writing are the names Dolly, Carry, Alex, and Willy - four of Herschel's young family of children. Alex and Willy both went on to be distinguished scientists, and also photographers, Sir William J. Herschel being a promoter at the end of the century of the newly invented colour photography (see 39802, and also 13193). The upper part of this paper bears an inscription that has been brought out by heavily brushing with a chemical that gives a brown stain. The lower part, with the children's names, has a different appearance, and a lilac or light mauve colouring. This and the fact that the pertinent Latin motto Ars Magna Lucis et Umbrae is the line written above the names might suggest that a photographic process has been used here.

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