John F W Herschel's notebook, 1839.
© Science Museum / Science & Society Picture Library
Pages from an experimental notebook of Sir John Herschel (1792-1871) dated 11-13 February 1839. Herschel was an English astronomer, mathematician and chemist, and was the son of Sir William Herschel. The page shows photogenic drawing negatives with notes on 'fixing' photographs. Herschel introduced 'hypo' (thiosulphat) as a photographic fixative and discovered the cyanotype or blueprint proces in 1842.