Stereo viewer used to look at stereo photo cards
Many photographers took special "stereo views" that made their photos look three-dimensional. First they took the pictures with a camera that had two lenses (like binoculars). The pictures were printed side-by-side on a stereo card. People used a viewer called a "stereopticon" to blended the two pictures into one three-dimensional image.
Stereo views were a new fad in 1889. Photographers liked being able to sell their pictures this way, since printing photos in newspapers, magazines, or books was still difficult and expensive. People bought thousands of stereo cards of the Johnstown Flood. Three-dimensional pictures really helped people understand how bad the flood damage was.