Four leaves by Herschel, 1839.
© National Media Museum / Science & Society Picture Library
Water fixed photogenic drawing negative of four leaves. Sir John Herschel (1792-1871), English astronomer, mathematician and chemist and the son of Sir William Herschel. Herschel introduced 'hypo' (thiosulphate) as a photographic fixerative and discovered the cyanotype or blueprint process in 1842.