FROM THE IEE ARCHIVES

Written in light One of the most striking images in the IEE Archives Image exposing the plate to mercury fumes and fixed with Collection is a calotype photograph of Michael Faraday and sodium chloride. his wife Sarah that was taken in the 1840s. The next stage in the development of the art was the The term photography, from the Greek for light and calotype, as seen here. Strictly speaking, the term calotype writing, was first used by Sir John Herschel in 1839, the refers only to the negative, and the positive image is a salted year in which photographic processes became public. The paper print, but calotype is usually taken to refer to the principles on which photography is based, however, had whole process. Patented by Fox Talbot in 1841, the calotype been known about for centuries. A photograph is the was produced by sensitising good quality writing paper in production of an image by the reaction of metallic salts to solutions of potassium iodide and silver nitrate. The paper light. Aristotle (384-322 BC) had carried out experiments was then placed in a camera and exposed to sunlight. The on the effects of light on certain pigments and salts, and the latent image was developed in solutions of gallic acid and camera obscura (or dark room) was used in art during the silver nitrate. After processing, the paper was usually waxed Renaissance. to make it translucent and this could then be used as a By the beginning of the 19th century, scientists such as negative from which multiple salted paper prints could be Humphry Davy were able to produce photogenic drawings, made. but they were unable to ‘fix’ the image, which had to be looked at in very dim light. As with so many technological developments, several people arrived at solutions to the fixing problem at approximately the same time. In 1839, William Henry Fox Talbot patented a method of producing photogenic drawings, which were stabilised with metal halides. The best known of the inventors of photography, however, is probably Louis Daguerre. A daguerrotype consisted of a silvered copper plate that was sensitised over fumes of iodine and exposed in a camera. The The calotype process that was used to create this 1840s photograph of Michael Faraday and his image was developed by wife Sarah was patented by William Henry Fox Talbot in 1841

10 IEE REVIEW NOVEMBER 2000