Stereoviews, also called stereocards, were popular from about 1840 to 1930. Two pictures were taken from slightly different locations. Then they were mounted on a stiff card and viewed through a stereoscope with two lenses. Just like the stereo movies made today, the method of photography made the pictures look 3-D. Collector associations include the Magic Lantern Society of the United States & Canada, the National Stereoscopic Association, and photographic groups in many cities. For more information and prices, see Kovels.com/Price-Guide/Stereo-Cards.html.