The 4th dimension of 3-D.
D003867:Depth Perception; D004739:England; D049672:History, 19th Century; D006801:Humans; D008853:Microscopy / Q000266:history / Q000295:instrumentation; D055095:Optics and Photonics / Q000266:history* / Q000295:instrumentation; D014785:Vision, Ocular