NIEPCE'S APPLICATION OF THE BITUMEN PROCESS.
A metal plate was coated with a solu tion of bitumen, and allowed to dry. It was then exposed for several hours to the image formed inside a camera obscura, the result being that the parts affected by light were rendered insoluble, while the rest remained unaltered. There is no change perceptible to the eye, but on the application of oil of lavender, those parts unaffected by light are dissolved away, while the exposed portions remain. Niepce had, in fact, obtained a photo graph, or, as he called it, a heliograph= a picture traced upon the metal plate in varying gradations of asphalt. He was not long in applying his discovery to the purpose for which it was primarily in tended, and was soon able, by the use of acid, to etch out the image obtained in this manner on copper and other metals.
It is, indeed, an improvement on the method of heliography, or sun drawing, invented by Niepce's nephew, Niepce de Saint-Victor, which is now principally in use for the printing of bank-notes and cheques. Heliography was the term ap plied to his process by Niepce himself ; nowadays the word is used with quite a different meaning. The picture formed by the bitumen on the metal plates was a negative one, and although suitable for etching, was not satisfactory for viewing. Niepce overcame this by blackening the metal plate with iodine. In 1829 Niepce entered into partnership, which lasted till his death in 1833, with Louis Jacques Mande Daguerre, who made many im provements in the heliographic process. Daguerre was born at Cormeilles, near Paris, in 1789, and died in 1851.