Keeler took the photograph seen at left in autumn 1899 to study the edge-on "spiral nebula" NGC 891 (the bright streak at center), but the plate, and scores of others like it, revealed much more. An enlarged corner of the photograph (outlined at left) is seen above. It shows a few of the numerous unknown nebulae—now understood to be galaxies—which appeared unexpectedly in the backgrounds of many of the approximately one hundred plates taken for Keeler's program. The discovery that these "nebulae" were so numerous and clearly a significant component of the universe was of great importance. (See the previous exhibit item for more about the plate itself.)

Volume VIII of the Publications of the Lick Observatory, in which the results of Keeler's photographic program were posthumously published, lists 744 new "nebulae" discovered on the plates, but he estimated the number within the Crossley's reach to be about 120,000. (By the time of publication in 1908, the figure had been revised to about a half million. With modern detectors the figure is much higher still.)

Lick Historical Collections, photographic plate archive.