Sir Henry Roscoe (1833-1915), British chemist. He was born in London and educated in liverpool and at University College London, He then travelled to work under Robert Bunsen at Heidelberg. Bunsen and Roscoe carried out a memorable series of researches on photochemistry. They are reputed to have invented flash photography in 1864, using powdered magnesium, and created the reciprocity law that states that the density of a photographic image is proportional to the inverse logarithm of the relative exposure. In 1857 Roscoe had been appointed professor of chemistry at Owens college, Manchester. In 1867 he began his work on vanadium, inventing a new process for purifying the metal. He also worked on the chemistry of niobium, tungsten and uranium and the solubility of ammonia. He was knighted in 1884 and from 1896-1902 was vice-chancellor of the University of London.