In 1840, William Henry Fox Talbot develops a negative-positive system, in another procedure called calotype. It consisted of obtaining a paper negative, which then by contact was positive on another sheet of paper. The paper was moistened in an acid solution of silver nitrate, before and after exposure, and before being fixed. It was the invention of the photographic copy, since a single negative could lead to several positives.
In 1842, the English astronomer and chemist John Herschel introduces the process called cyanotype. He was also the first to apply the terms “positive and negative” to photographic images. In 1819, Herschel discovered the solvent power of sodium hyposulphite around insoluble silver salts, setting a precedent for its use as a fixing agent in photography. He informed Talbot and Daguerre of his discovery in 1839, and that this could be used to permanently fix images. He made the first crystal negative in late 1839.