Keystone Eye Comfort and Depth-Perception Series E.C. 112-33293: A Peaceful Spot in the Ozarks
From one of several Keystone View Co. stereoscopic image series designed to strengthen the eye. Based on the late 19th-century research of French scientist Louis Émile Javal and popularized as optical diagnostic and therapeutic tools in the 1930s and '40s. Keystone View Company (Meadville, PA) published several variations, including the "Eye-Skill Series," "Comfort and Depth Perception Series," and "Telebinocular Tests of Visual Efficiency."