ABSTRACT
The basis for the stereoscope’s eect is an aspect of visual experience known as stereopsis (the ability to perceive depth by virtue of binocular vision) and is founded on our ability to combine the slightly diering views aorded by the left and right eye.2 The rst clearly articulated account of stereopsis appeared in 1838 in the form of a paper delivered to the Royal Society by the then chair of Experimental Physics at King’s College London, Charles Wheatstone.3 When looking at objects in the far distance, he pointed out, the axes of both eyes will be parallel and no perceptible dierence between the views from the left and the right eye will be observed. However, as the eyes converge on objects at closer proximity, the relative disparity between the two views will increase and signicantly dierent ‘perspective projections’ will be created. It was therefore possible, as Wheatstone discovered, to reproduce the visual impression of a solid object simply by replicating these perspectives. His invention, the reflecting stereoscope (Figures 21.1 and 21.2), produced the desired result by virtue of a pair of mirrors set at forty-ve degrees directly in front of the viewer. Wooden easels with corresponding perspective drawings mounted on them, constructed as if viewed from the left and right eyes respectively, were positioned at either end of the device. Each picture was therefore reected in one of the
two mirrors and directed towards either the left or right eye of the viewer.4 Rudimentary though this apparatus was, by presenting images in this manner he found he was able to articially stimulate stereopsis and in the process create a pronounced sense of three-dimensionality in the image. As a consequence, he not only provided the key to a new understanding of threedimensional visual experience but also drew attention to an aspect of vision that had always existed but of which, arguably, the viewing subject had previously been unaware.5
