Phantascope Phantascope
Home Button
Contact Button  

A combined pocket size Hyperscope & Psudoscope,
and more.

Discover the Eyebenda

Find out about the Eyebenda spacer
Further Reading

Scientific American

The Hyperscope and Pseudoscope Aid Experiments on
Three-Dimensional Vision

by Jearl Walker

November, 1986

How is it that the visual system sees things three-dimensionally when the image on the retina is two-dimensional? The reason is that one interprets a variety of cues in the retinal images to create a perception of depth in a scene. Terry Pope of the University of Reading has devised two instruments that alter the cues so that he can do experiments on the perception of three dimensions.

Figure 1: Terry Pope's hyperscope and pseudoscope
Figure 1: Terry Pope's Hyperscope (left) and
Pseudoscope (right)

The cues about distance and depth can be grouped into five categories: convergence, retinal disparity, accommodation, motion parallax and pictorial. Convergence involves the angle between the lines of sight from each eye when you look directly at an object. Retinal disparity involves the difference in the position of an image on the two retinas. Accommodation is a change in the shape of the eye's lens in order to focus an object onto the retina. Motion parallax is the relative motion of near and far objects through your field of view when you move or the objects move. Pictorial cues involve the information about depth that can be perceived even in a flat painting. Included are lines of perspective, the blocking of one object by another, shadows and shading and the variation in the density of textures with distance.

Convergence and retinal disparity play a role in most perceptions of three dimensions. They invoke the concept of the visual axis, or line of sight, which is an imaginary line connecting an object with its image on the retina when you look directly at the object. Suppose you look directly at an object A. Its image lies on the visual axis, and so on the same part of the retina in each eye, enabling the brain to fuse the two views into a single perception. The angle between the two visual axes is called the angle of convergence. It is related to the angle through which the eyes must turn in order to direct their axes at A. The visual system associates that angle with distance to the object: the larger the angle is, the closer the object seems to be.

When you look directly at A, the images of a more distant object B are at different places on each retina. The visual system recognizes this disparity as a cue to the depth between A and B. The recognition can also be explained in terms of convergence angles. If you look directly at B, the angle between the visual axes is smaller than it was for A. Therefore B must be farther away than A.

Figure 2: Retinal disparity
Figure 2: Retinal disparity
Retinal disparity depends partly on the separation between the two eyes, which is about six centimeters. Pope set out to explore what happens to perception if the separation is changed. One of his instruments, the hyperscope, effectively increases the separation to about 20 centimeters by means of reflections from mirrors. Similar instruments were studied by Charles Wheatstone and David Brewster in the 19th century.

The increase in the effective distance between the eyes increases the retinal disparity of images formed on the retinas and the difference in convergence angles when you look from one object to another at a different distance. Suppose you look at A through the hyperscope while B is also in view. The new disparity of separation between the images of the two objects 9 on the retinas forces you to perceive greater depth between them. You also perceive greater depth because the difference in convergence angles for the objects is now greater.

The hyperscope also alters the apparent height and width of nearby objects. In normal vision you are accustomed to a certain relation between the size of an object's image on the retina and the object's distance, as implied by the convergence of the eyes when you look at it. Seen through the hyperscope, an object looks smaller because the angle of convergence required to see it through the mirrors is larger than normal.

Figure 3: Pope's Rhombus
Figure 3:
Pope's Rhombus
Many other familiar objects take on a strange appearance through the hyperscope. For example, a person's face looks thinner and seems to have a prominent nose. All the objects immediately return to their normal appearance if you close one eye while still looking through the instrument with the other one. Because you are no longer able to compare retinal disparity or convergence angles between the eyes, you are left with only the pictorial cues about depth.

Another of Pope's instruments, the pseudoscope, makes use of mirrors to switch what the eyes are seeing. The exchange reverses the cues about distance from retinal disparity, sometimes causing a distant object to seem closer than a nearer one. The exchange of depth is most vivid for me when I look through the pseudoscope at complex arrays such as trees or brush. Branches at the rear of a tree seem closer than branches at the front. The sight is eerie because I realize that the front branches partially block my view of the rear branches. Depth is also inverted when I look at an object that can easily be reversed mentally. For example, a pot hung bottom out on the kitchen wall suddenly appears to bulge inward rather than outward.

Pope has made several constructions of transparent plastic that seem to move surprisingly when a pseudoscope inverts them. One is a rhombus consisting of two plastic parallelograms held together by four metal rods. The rhombus is suspended by a thin wire. Propped on top of the rhombus is a band of alternating green and black stripes. One of the rods passes through the band to hold it in place.

Figure 4: Color-stereoscopic phenomenon
Figure 4:
Color-stereoscopic phenomenon
I suspended this device from a ceiling lamp fixture, rotated the rhombus to twist the wire, released it and then viewed it through the pseudoscope from a distance of about five meters. Initially the rhombus and the band rotated together, first one way and then the other as the wire twisted and untwisted. Suddenly the rhombus inverted front for rear. Thereafter it and the band appeared to rotate in opposite directions. Although I knew the two objects were firmly connected, I could not shake the illusion of opposite rotation until I closed one eye.


THE COLOUR STEREOSCOPIC EFFECT. B. N. Kishto in Vision Research, Vol. 5, pages 313-329; June, 1965.

THE EFFECT OF PUPIL SIZE VARIATIONS ON THE COLOUR STEREOSCOPIC PHENOMENON. Jon Martin Sundet in Vision Research, Vol.12, pages 1027-1032; May, 1972.

BREWSTER AND WHEATSTONE ON VlSION. Edited by Nicholas J. Wade. Academic Press, 1983.

Suppliers and Organizations

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800