Keystone Eye Comfort and Depth-Perception Series E.C. 106-33287
"Keystone Eye Comfort and Depth-Perception Series: For the Development of Comfortable Vision and a High Degree of Depth Perception"
From one of several Keystone View Co. stereoscopic image series 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."
Stereoscopic albumen print, ca. 1940. Card slightly bowed, otherwise VG; 3.5 X 7 inches. Housed in mylar sleeve for safekeeping.