"The history of photography is often told as the triumph of one technology over another," says Sarah Greenough, one of the curators of a new photography show opening at the National Gallery today. "But this exhibition tells a different story."

Running through May 4, Impressed by Light, is a collection of British photographs from paper negatives taken between 1840 and 1860. Called calotypes, the images created using this early technique were believed to have been rendered immediately obsolete upon the introduction of an "improved" collodion-on-glass negative process in 1851.

On Exhibit

Impressed by Light runs through May 4 at National Gallery, 4th Street and Constitution Avenue Northwest, Washington. Free. 202-737-4215 or www.nga.gov