Bunsen became professor of chemistry at the University of Heidelberg in 1852, and it was there he met the British student Henry Enfield Roscoe, who became his collaborator. They published a series of articles from 1854 to 1859 on the chemical action of light in photometry, acti-nometry and spectrum analysis. Considered by many historians to mark the foundation of photochemistry, their work resulted in the formulation of the Bunsen and Roscoe Law, or the Reciprocity Law. The law stated that the amount of a photochemical reaction depended upon the light energy absorbed, thus relating to the development of densities in a photographic emulsion. Later research by Schwarzschild, Sheppard, Mees and others showed that the rule applied chiefly to moderate intensities of illumination. Bunsen and Roscoe also discovered that magnesium wire or ribbon could be used as a source of photo¬graphic light, and presented papers on this work in 1859 and 1864. They invented an apparatus for burning magnesium wire wound on spools and moved by clockwork, the end of the wire being ignited by the flame of an alcohol lamp.
Bunsen's other inventions included a carbon- zinc electric cell (1841), grease-spot photometer (1844), filter pump (1868), ice calorimeter (1870) and vapor calorimeter (1887). He gave his name to the Bunsen burner, but contributed in only a minor way to its development.
In 1860, Bunsen was elected a foreign member of the Royal Swedish Academy of Sciences.