There are some terrific photographers in the Museum of London’s current exhibition, London Street Photography. The photographers range from John Thomson, who was a 19th-century pioneer in this genre when long exposure times and bulky cameras made such photographs minor miracles, to Nick Turpin, who founded the website In-Public in 2000 to give contemporary practitioners a place to congregate and keep the genre going. Turpin’s is classic work that catches the chance juxtapositions and silly associations on which hand-camera street photography has thrived since Henri Cartier-Bresson was declared a boy genius by the Surrealists in the 1930s. Thomson was an inventor of the genre because he risked his laborious equipment on frivolous moments, like the one at which a little girl came skipping by “Hookey Alf,” a worker who had lost a forearm in an industrial accident. Thomson was prescient enough to realize that instead of ruining an exposure, some motion blur or other aberration could be the very making of the picture. He was an aberration himself, a motion-blurred leap into the future of photography that would contain Cartier-Bresson, Walker Evans and Robert Frank.