An Albert Einstein signed letter featuring his handwritten calculations has sold as part of RR Auction's latest online sale.
The letter, written to the Jewish Student Union in Germany, sold for $37,982.
Science collectibles such as this are popular on the current market, the $6m record sale of Francis Crick's DNA letter at Christie's in April supplying a particular highlight.
The two-page handwritten Einstein letter, dated December 25, 1925, reads: "To the Jewish Student Union in Germany, I wish your conference the best of success. I would gladly be present myself, but unfortunately urgent work prevents me from the appearance. I always wish my utmost support, to demand your goals."
Einstein has included a page of his calculations, giving students a glimpse into his attempts to generalise his geometric theory of gravitation, the project that followed Einstein's famous research on general relativity.
Einstein was lecturing in Berlin at the time he wrote the letter, and had previously served as the first president of the World Union of Jewish Students, which was established to address the issue of an unjust quota system that greatly limited the number of Jewish students admitted to European universities.
The letter was followed by a signed circa 1842 letter from Charles Darwin, which sold for $37,541 due to his rare reference to his historic first voyage on board HMS Beagle.
Written to his close friend John Herbert, the letter features Darwin's recommendations for the "best bird-preservers". He also doubts that the birds of paradise that Herbert refers to are genuine, joking: "Tell that to the Marines, as we used to say on board the Beagle."
He finished the letter: "Farewell my ancient amigo. Ever yours, C. Darwin."
Join the hot market for science collectibles with this Thomas Edison autographed document, which is signed by the great inventor as head of the Edison Spanish Colonial Light Company.