A comprehensive investigation of an optical instrument that produces apparent three-dimensional relief by uniting two flat images, covering its historical development, varieties, and mechanical construction. It explains monocular and binocular vision, the underlying optics and geometry of image fusion, and detailed descriptions of ocular, reflecting, and lenticular forms. Practical guidance is given for photographing, sizing, positioning, and illuminating paired pictures, and for adapting single images into stereoscopic views. The text surveys applications to painting, sculpture, architecture, engineering, natural history, education, and entertainment, and it discusses optical fallacies, common difficulties in use, and techniques to improve reproducibility and visual accuracy.