A man visits the exhibition "The Eye of the Sun: Nineteenth-Century Photographs from the National Gallery of Art" in Washington D.C., the United States, on Oct. 4, 2019. The exhibition marks the 180th anniversary of photography's introduction to the world in 1839 and presents some 140 photographs offering an in-depth look at the development of the medium throughout its first 50 years. It will last until Dec. 1. (Xinhua/Liu Jie)