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Binocular instruments were devised long before the basis of stereoscopic vision was understood. In the 17th century binocular microscopes and telescopes were made in France and Italy. They were simply pairs of monocular devices mounted side-by-side, usually with some means of adjusting the convergence of their axes. These instruments were made in the belief that vision with two eyes would be better than with one, but neither utilized the essentially binocular characteristic of retinal disparity. Because the eyes are separated by around 2 - 2½" (6 - 6.5cm) the images cast on each retina are different or disparate. The extent of the difference depends ut5on the interocular separation and the distance of the object from the eyes: the closer the object the greater the disparity. The relationship between retinal disparity and stereoscopic vision was not appreciated until the 19th century, when the physicist Sir Charles Wheatstone invented the stereoscope. Before that time it was considered that equivalent images in each eye were combined, but any differences in the two retinal images were thought to produce double vision. This double vision can easily be demonstrated by holding up the index finger of each hand in the straight-ahead but at different distances from the eyes - say at 8" and 16" (20 and 40 cm). When you fixate on the near finger you will see two far fingers, and vice versa! In these cases the disparities are too large to be combined, and double vision does indeed occur, but for smaller disparities stereoscopic depth, rather than double vision, results. This is what is happening nearly all the time when we have both eyes open, and it is easy to check on the retinal disparities by closing one eye and then the other, and comparing the relative locations of objects. When we vary our fixation from a far object to a near one the eyes rotates in opposite directions - they converge. This changes the horizontal disparities in the eyes. The eyes do not rotate in contrary directions vertically, one up and the other down; therefore, the important disparities to which we respond are horizontal ones.

Wheatstone found away of presenting these different projections separately to each eye. His stereoscope, which was first demonstrated publicly in 1838, consisted of two mirrors at right angles to one another which reflected slightly different drawings to each eye. The depth that was seen depended upon the differences in the images (retinal disparity) and the direction the eyes were pointing (convergence). So the mirror stereoscope was the first truly binocular instrument. Following its announcement a wide range of optical means for separating the images to the two eyes were demonstrated, most notably by the Scottish physicist Sir David Brewster; stereoscopic pictures could be combined using one or two mirrors, prisms, or half-lenses. Brewster's popular stereoscope, first made in 1849, used a single lens that was split into halves and mounted so that they acted like prisms and lenses; the cut parts were placed away from the centre so that they were like base-out prisms. The two pictures that were mounted side-by-side were refracted by the prismatic component so that each appeared to come from a central location; the lens property magnified the images. The invention of the stereoscope coincided with that of photography, and the popular potential of the former was realised when the two were combined in the mid-nineteenth century. At first, a single camera was used to take two photographs from different lateral positions, but there were soon several designs of stereoscopic cameras on the market. Brewster's stereoscope was more suited to use with stereophotographs, and so became more popular than Wheatstone's model.

An exaggerated impression of depth would be produced by taking two photographs with the cameras separated by distances far greater than the 2½" (6 - 6.5 cm) that separates the eyes. By this means depth could be seen in stereoscopic pictures of buildings or landscapes that would not be visible to the naked eyes when viewing them. Extending the effective interocular separation by adding two more mirrors, as in the hyperscope, also produces an exaggerated impression of depth by enhancing the retinal disparities. The German physicist Hermann von Helmholtz first constructed such an instrument in 1857, and he called it a telestereoscope. The hyperscope increases the separation between the eyes by a factor of seven. This means that objects viewed from several metres distance will have the disparities normally associated with much closer ones. On the other hand, if you look at objects that are too close the retinal disparities can be so large that they are seen double rather than in depth. The Hyperscope produces very compelling effects when directed towards people or objects several metres away. For example, if you look at a person who is facing you, and they point their arm towards you, then it will appear amazingly long; if it is moved about it will seem to be made of elastic. It is even more startling when the person stretches their neck in your direction - it looks as though it will become separated from the torso! You will be able to discover many more remarkable effects when you look at the world through the Hyperscope.

Returning to the simple demonstration with two fingers at different distances from the eyes: if you fixate on the near finger and then close each eye in turn, you will notice that the far one appears to the right with the right eye and to the left with the left eye. This is called uncrossed disparity, and it is the characteristic of projections from all objects farther away than the point of fixation. The reverse occurs when you fixate on the far finger, and it produces crossed disparity. All projections from objects nearer than the fixation point yield crossed disparities. Wheatstone found that he could reverse the depth seen in a stereopair by transposing the left and right images. That is, the picture that would normally be presented to the right eye was viewed by the left. He was reversing the retinal disparities by this procedure so that what had previously looked farther away now looked closer. In 1852 he made a simple instrument from two prisms, called a pseudoscope, which reversed the disparities when viewing a normal scene. This resulted in some very peculiar experiences - the insides of teacups appearing to protrude, busts looking like hollow masks, pictures on the wall seeming to be embedded in it. However, the effects take some time to occur, because there is some conflict between the previous and present appearance of the same objects. As Wheatstone said "I know of nothing more wonderful, among the phenomena of perception, than the spontaneous occurrence of these two very different ideas in the mind, while all external circumstances remain precisely the same."

One of the disadvantages of Wheatstone's prismatic pseudoscope is that it reverses the images from left to right; another shortcoming is that the the apparent depth reversals can take some time to occur, especially when looking at familiar things. For this reason it is often preferable initially to look at some relatively isolated object, like a lampshade against a plain ceiling. At first it will look a little strange, but with more prolonged viewing it will probably appear reversed in depth, so that what is actually closer to you will look farther away. An even more dramatic effect is produced when a slide of some detailed texture is projected onto an uneven or curved source, such as a large unfurled roll of white paper, particularly if someone else can hold it and move it about. The impression of the three-dimensional surfaces is more clearly defined by the reversed disparities in this case, because there are fewer alternative cues to distance, like interposition, that can conflict with them. As Wheatstone said "With the pseudoscope we have a glance, as it were, into another visible world, in which external objects and our internal perceptions have no longer their habitual relation with each other."

Brewster,B Description of several new and simple stereoscopes for exhibiting,as solids, one or more representations of them on a plane. Transactions of the Royal Scottish Society of Art, 1857, 3 , 247-259.

Clay,R.S. The stereoscope.Transactions of the Optical Society 1927-28,29 149-166

Dove,H.W. Breschreeibung uehrerer Prismeustereoskope und eines einfachen Spiegelstereoskops. Poggendorff's Annden der Physik und Chemic, 1951, 83, 183 - 189.

Gill, A.T Early stereoscopes. The Photographic Journal, 1969, 109, 546-559, 641-651. (3 articles)

Jones, J. Wonders of the Stereoscope. London: Cape, 1976.

Professor Nicholas J Wade