The structure of Lippmann heliochromes: Cajal and the 1908 Nobel Prize in Physics.

The 1908 Nobel Prize in Physics was awarded to Luxembourgeois Gabriel Lippmann (1848-1921), Professor of Mathematical and Experimental Physics at la Sorbonne, for his method of reproducing colors photographically based on the theory of wave interference. In the preceding several years, the eminent neurohistologist - and avid photographer - Santiago Ramón y Cajal (1852-1934) had been experimenting with Lippmann heliochromes, studying under the microscope the structure of the laminae of Zenker that produce mixed colors, and especially white. Those studies led to a series of technical papers by Cajal, the culmination being an article published 100 years ago in the Annual Report of the Board of Regents of the Smithsonian Institution in Washington, DC. A few years later, Cajal published Photography in Colors, his classic monograph on the physicochemical principles of the 'art of Daguerre,' bearing further testimony to his exuberant productivity, far-reaching interests, and scientific genius. The present article reflects on the workings of the mind of Cajal and his fundamental knowledge that was a precondition for his success in neurohistology. It highlights the links between the early photographic studies of Cajal and Lippmann, masters of the biological and physical sciences, respectively. Special emphasis is placed on Lippmann's discovery of heliochromes and the microscopic analyses performed on them by Cajal, including elements from relevant contemporary studies and discoveries.