The birth of Stereoscopy: Wheatstone on Binocular Vision 1838, original source
Back in 1838 the concept of binocular vision had not yet been explored or written about anywhere. It was a scientist in his mid 30s who not only described the phenomenon later called stereopsis but also constructed a device to view two flat images in 3D which he called a stereoscope. This is especially remarkable as photography was not invented until one year later. Charles Wheatstone’s observations were based only on drawings. Most of these drawings are based on horizontal mirroring which is why we call them mirror stereos today. Read Wheatstone’s original source here.