Stereoscopic photography was popular in the late 19th and early 20th centuries. This kind of photography involves placing two photographic images side by side; when one views them through a lens called a stereoscope, the side-by-side photographs create the illusion of a three dimensional image. Richard M. Dennis collected thousands of these photographic images throughout the 20th century. In 1939, Dennis sold his collection - a total of 35,000 images - to the New York Public Library. Continuing to collect stereoscopic images, he then donated an additional 35,000 images in 1980. Today, the New York Public Library has digitized these images and made them available through its substantial Digital Collections. These photographs were taken between 1850-1930 and depict cities and landmarks across the United States. Website visitors can search the collection or browse for certain stereoscopic views by date or geographic location. Some items have also been added to the Stereogranimator, where readers may view, create, and share stereographs as they were intended, in 3D form. This collection provides a fascinating glimpse into American history, urban development, and the history of photography.