Under penalty of losing funding, the Smithsonian has been instructed by Congress not to mess with the mainstream consensus about Hiroshima and Nagasaki: that the A-bombings were necessary to save lives that would have been lost in an American invasion of Japan. That view was not exactly a consensus even in 1945, however: Fifty years ago today, 68 Manhattan Project scientists signed a petition calling for a demonstration of the bomb on an unpopulated site before it was used on a military target. Two of those signers, Ralph Lapp and John A. Simpson, will discuss the unsuccessful appeal; their discussion will be moderated by William Lanouette, who wrote a biography of Leo Szilard, the man who initiated the petition. At noon at the National Archives, Room 105, 7th & Pennsylvania Ave. NW. FREE. (202) 501-5000. (MJ)