This is a wood stereoviewer with folding handle and a set of 23 sterocards to demonstrate the complex 3-D structure of various crystal types. The viewer has two lenses at a set distance in the wooden hood. The pictures can be moved horizontally on the slide holder in order to focus them. These devices became popular in the middle of the 19th century. This Holmes type stereoscope was named after its inventor, Oliver Wendell Holmes (1860). The cards show typical diagrams of Hemihiedral and Holohedral Forms of crystals, as well as Cubic, Rhombic, Monoclinic and Tetragonal Crystal Systems. For each system there are several examples of different types of crystals. See Edward S. Dana and William E. Ford: A Textbook of Mineralogy, 1949.