On the late invention of the stereoscope.

It was not until 1838, when Wheatstone published his account of the stereoscope, that stereoscopic depth perception entered into the body of binocular phenomena. It is argued that the stereoscope was not invented earlier because the phenomenon of stereopsis based on disparity had not been adequately described. This was the case despite the fact that there had been earlier descriptions of tasks that could be performed better with two eyes than with one; the perceptual deficits attendant upon the loss of one eye had been remarked upon; analyses of the projections to each eye were commonplace, and binocular disparities were accurately illustrated; moreover, binocular microscopes and telescopes had been made over a century earlier. Theories of binocular vision were generally confined to accounting for singleness of vision with two eyes, and the concepts employed to account for this were visible direction, corresponding retinal points, and union in the brain. The application of these concepts inhibited any consideration of disparities, other than for yielding diplopia. When perception of the third dimension was addressed by Berkeley at the beginning of the eighteenth century, it was in the context of monocular vision and binocular convergence. Thereafter visual direction became the province for binocular vision and it was analysed in terms of geometrical optics, whereas visual distance was examined in the context of learned associations between vision and touch. This artificial division was challenged initially with respect to visual direction and later with respect to stereopsis. An additional factor delaying the invention of the stereoscope was that experiments on binocular vision generally involved abnormal convergence on extended objects. Wheatstone's accidental observation of stereopsis was under artificial conditions in which disparity alone defined the binocular depth perceived. Once invented the stereoscope was enthusiastically embraced by students of vision. It is suggested that the ease with which retinal disparity could be manipulated in stereopairs has led to an exaggeration of its importance in space perception. "The appearances, which are by this simple instrument rendered so obvious, may be easily inferred from the established laws of perspective; for the same object in relief is, when viewed by a different eye, seen from the two points of sight at a distance from each other equal to the line joining the two eyes. Yet they seem to have escaped the attention of every philosopher and artist who has treated of the subjects of vision and perspective." (Wheatstone 1838, page 371).