He documented the poverty previously hidden in darkness using a magnesium powder to produce a flash. His work led to changes in the city and prompted then-Police Commissioner Theodore Roosevelt ...

Flash photography can be a mystery ... and shows star trails and city lights on Earth some 250 miles below.

Jacob Riis, a writer and photographer who exposed poverty in late 19th century New York City, used crude tools ... In the 1890s, "people were new to flash photography, and when [Riis] set it ...