In 1931 Einstein was invited to speak at Oxford to give a series of three lectures on relativity. This blackboard was preserved from the second lecture on 16th May 1931 and is now displayed in the Museum of the History of Science.
From the plaque beneath the blackboard: "The first three lines establish an equation for D, the measure of expansion in the universe. The lower four lines provide numerical values for the expansion, density, radius and age of the universe."
(Originally published: http://www.flickr.com/photos/garrettc/2172850095/)