The first scientist honored on an American stamp was John James Audubon in 1940. Albert Einstein has been on two, in 1966 and 1979. Last month at Yale University the U.S. Postal Service issued a block of four stamps commemorating the careers of a mathematician, John von Neumann; a pioneering thermodynamicist, Josiah Willard Gibbs; and two Nobel winners, geneticist Barbara McClintock and physicist Richard Feynman. The scientists were chosen from a field of 50,000 nominations that the postal service gets each year. To qualify, they had to be Americans and dead for at least a decade. And, of course, they had to have had notable careers, “instrumental to the American experience.”