In four photographs of Albert Einstein, taken over a 30-year span between 1911 and 1942 and reproduced in Silvan Schweber's "Einstein & Oppenheimer: The Meaning of Genius" (Harvard, 432 pages, $29.95), he positions himself, whether in a group or alone, so that his left hand is caught by the camera. He holds that hand in a distinctive gesture, with his thumb and forefinger joined to form a little ellipse. Though he tends to face away from the camera, as though indifferent to appearances, he is clearly at pains to keep that left hand visible. The gesture is as much a signal as a symbol.
