Einstein's theory of Brownian motion allowed Jean Baptiste Perrin to demonstrate the existence of atoms in 1908. Perrin, who won the 1926 Nobel Prize for Physics for this work, is sitting sixth from the left, leaning forward, while Einstein is standing second from the right. This picture was taken at the first Solvay Congress in Brussels in 1911.

Credit: Benjamin Couprie, Institut International de Physique Solvay/AIP Emilio Segre Visual Archives