Last year I built a simple version of the wonderful, first ever, stereoscope. I used it to demonstrate the principles of our binocular vision, just as its inventor, Charles Wheatstone, had used it to work out those principles. This satisfied my own curiosity; but also meant that I could explain to my colleagues how it is possible to create the impression of a three dimensional world using photographs.
This is a tricky thing to photograph – because of its two mirrors! – but I tried my best. probably ought to have ironed the backdrop…
On 21 June 1838 Charles Wheatstone, Professor of Experimental Philosophy at King’s College, London, presented a paper to the assembled membership of the Royal Society. Wheatstone was dissatisfied with the various theories that tried to explain how people see a single image of the world around them.
We have two eyes, and therefore receive two separate images of our surroundings. How come we only see one world around us, not two? Wheatstone had undertaken a range of experiments with the aim of understanding sight.
He began by observing that when you look at an object at a distance – the shed at the bottom of your garden, for example – it looks just the same whether you view it with both eyes or with only one. The two separate lines of sight between each eye and the distant object are, to all intents and purposes, parallel; each eye sees exactly the same image.
When you look at a nearby object, however, the two lines of sight converge; so each eye sees a different perspective of the object. Leonardo Da Vinci had made a similar observation about looking at things close to:
Try this experiment to see this working:
 place a die at the far end of the table;
 kneel at the other end of the table and look straight along the table top to the die, with both eyes open;
 keeping your head very still, look at the die with first one eye covered, and then the other.
The die should appear the same in all three views. At this distance, the lines of sight are parallel. The die will look more like it is flat, and less like a cube:
 now bring the die to within about 15 cm of your face;
 keeping your head very still, look along the table at the die first with both eyes, then with one eye covered, and then the other.
This time, you should always see the front of the die: but with your left eye alone you should also see dots on the left-hand face of the die; and with your right eye alone you should also see dots on the right-hand face of the die. It will look more like the die is a cube:
Wheatstone was the first person to observe that, when our lines of sight converge on a nearby object, we are seeing two dissimilar images. Therefore, he proposed, the brain perceives a three-dimensional object by means of these two different images.
Wheatstone then asked, “What would be the visual effect of simultaneously presenting to each eye, instead of the object itself, its projection on a plane surface as it appears to that eye?” That is, if your right eye could only see a drawing of the die as it looks in the right-hand photo above, and at the same time your left eye could only see a drawing of the die as it looks in the left-hand photo above, what would you perceive?
To address this question, he built the first ever stereoscope and made a set of drawings to use in it (including outlines of cubes, so I will continue to use this shape as the example). The stereoscope allowed Wheatstone to view separate images in each eye, at the same time.
With his face in front of the two angled mirrors (labelled A’ and A in the diagram above), he reflected the left-hand drawing (E’) into his left eye and the right-hand drawing (E) into his right eye. He saw a single, three-dimensional, cube.
This revealed that even though he was looking at a pair of two-dimensional drawings, he perceived a three-dimensional image. Wheatstone had proved that a three-dimensional view of the world results from our simultaneous perception of two different monocular images.
Wheatstone then went a step further. He had pairs of “skeleton figures” made; the outlines of three-dimensional objects, made in wire, which he put in place of the drawings in the stereoscope. One was a pair of wire cubes. He found he could place these to mimic the angles of his drawings of cubes, presenting two dissimilar images to each eye and thus observing a single, three-dimensional cube. However, he could also angle the wire cubes so that two identical images were presented to each eye; when he did this, there was no three-dimensional effect and it just looked like he was seeing a two-dimensional drawing.
Wheatstone concluded “that the most vivid belief of the solidity of an object of three dimensions arises from two different projections of it being simultaneously presented to the mind.”
You can see in the photo above how the two drawings reflect in the angled mirrors. The drawings are copies from Wheatstone’s original set. If you put your face in front of the mirrors, each of your eyes is presented with one drawing. The drawings show slightly different angles of the same object, so your brain perceives a single, three-dimensional image. This pair turns into a cone.
This is also why we can use pairs of photographs to create three-dimensional – “stereoscopic” – views.
Wheatstone, C. (1838) “Contributions to the Physiology of Vision – Part the First. One some remarkable, and hitherto unobserved, Phenomena of Binocular Vision.” Philosophical Transactions of the Royal Society of London 128:371-94
This post is the fourth in an occasional series called “Weird and Wonderful”.