Putting the stereoscope to use – Edouard Deville’s plotter

My last post was about Charles Wheatstone’s invention of the stereoscope, and the simple version that I made for demonstration purposes.   Following Wheatstone’s investigations into binocular vision and stereopsis – our perception of depth or three-dimensionality – his stereoscope was adapted for other uses.

First it evolved into a popular entertainment.   This development was made possible by Professor David Brewster’s 1849 proposal that two lenses, rather than two mirrors, could be used as the mechanism for directing the two images in the stereoscope to the viewer’s eyes.  Various models of this lenticular stereoscope were designed during the nineteenth-century, including simple, cheap, handheld devices that amused the masses with “magical” three-dimensional views of famous places, humourous scenes and, inevitably, pornography.

Brewster's stereoscope: slide a stereo pair of images into the back and look through the binoculars to see a 3D image (image via wikimedia commons).

Brewster’s stereoscope: slide a stereo pair of images into the back and look through the binoculars to see a 3D image (image via wikimedia commons).

In the meantime, the stereoscope was being put to more serious uses, one of which was map-making.   Known from the late-1880s as “photographic surveying”, this was the beginning of modern photogrammetry.

During the later nineteenth-century, geometrical methods of recording heights and distances from single ground photographs were being used for topographic mapping, most notably in Canada.   In a country with vast, un-mapped areas of mountainous terrain, it was expensive to send teams of surveyors out to make detailed maps using traditional methods.  Having triangulated an area to establish the essential framework, it was more economical to take panoramic photographs from carefully recorded camera stations and map the detail from each photo, back at the office.

The pioneers of photographic surveying, Aimé Laussedat and Edouard Deville, started by plotting from single photographs taken from positions that gave the best view of the area being mapped.   Using a pair of stereoscopic images (a stereogram), however, enables the precise recording of three-dimensional co-ordinates using the 3D image created in a stereoscope.

Making use of this advantage, in 1901 Carl Pulfrich (at the Carl Zeiss Works, Jena) and Henry Fourcade (in Cape Town, South Africa) independently produced designs for stereo-comparators.   These were in effect lenticular stereoscopes with additional instrumentation, so that X, Y and Z measurements could be made from a pair of ground photographs viewed in stereo.   The map-maker would make calculations using the three measurements from the stereogram and manually transfer the resulting point to a plot, then “join the dots” to draw up the contours of the landscape.

Pulfrich's stereo-comparator, 1904: the two photographs are placed on the plates, and viewed through the binoculars to reveal the 3D image in the mind of the viewer. The apparatus allows the viewer to measure dimensions across (X axis) and up-and-down (Y axis), but also height (Z axis).

A Pulfrich stereo-comparator photographed in 1904: the two photographs are placed on the plates, and viewed through the binoculars to reveal the 3D image in the mind of the viewer. The apparatus allows the viewer to measure dimensions across (X axis) and up-and-down (Y axis), but also height (Z axis) (image via Max Planck Institute for the History of Science).

A disadvantage of the stereo-comparator was that it involved these two operations: viewing the stereogram to take measurements; making a set of calculations and transferring the measurements to the map plot.   Between 1869 and 1902 Edouard Deville, Surveyor-General of Canada, invented a different device based on Wheatstone’s reflecting stereoscope with which contours could be “traced” directly from the stereogram.

Edouard Deville's plotter: at its heart is a Wheatstone mirror stereoscope. It also has the plotter, the device at the back of this image with a vertical plate and an upright pencil fixed in the base.

Edouard Deville’s plotter: at its heart is a Wheatstone mirror stereoscope. It also has the plotter, the device at the back of this image with a vertical plate and an upright pencil fixed in the base (image from Deville 1902).

The pair of stereo photographs, produced as transparencies, were mounted in frames B.   They are reflected by the mirrors mounted at A, and viewed by the map-maker through eye-holes in the propellor-shaped viewing stand.  The 3D image of the landscape is created in the viewer’s mind.   This image appears to fall onto the vertical plate of the plotter, C.   This plate has a tiny pin-prick of light at its centre.   By moving the plotter, this pin-prick is moved about the 3D image.   The pencil in the base of the plotter leaves marks on the plotting table.

Deville’s stereo-plotter was brilliant for its simplicity, but it had a significant drawback.   All the observations were made through the fixed viewing stand A, which therefore had to be changed for each map-maker in order to accommodate the different interval of each person’s eyes.   This also affected the scale of the plot, which would be slightly different from each operator.   Every finished plot would have to be laboriously re-drawn to the desired scale.

To make this invention easier to understand, I built a simple working model and demonstrated it using aerial photographs.   Here it is, fitted with the same pair of Wheatstone’s drawings from my previous blog post for you to compare.

Imagine this apparatus standing on a table.   You would be able to sit with your face in front of the two mirrors.  The drawings would appear to be one, 3D, image of a cone.   Switch on the light in the plotter.   Move the plotter until the pin-prick appears to touch the base of the cone.   Mark the plotting table with the pencil.   Pull the plotter towards you until the pin-prick reaches the top of the cone and mark the table.  Now you can measure how tall the cone is.

This is a very basic version of Deville’s plotter.  To create “see-through” mirrors I used sheets of the sticky-backed plastic used to tint car windows, because I didn’t have enough time to silver the glass myself.   Proper silvering would be a great improvement.  The pin-prick of light is a 3 amp LED run on a watch battery, assembled in a little cardboard box with a cardboard switch.  The glass sheets were scrap rescued from a 1950s window, and very thin and fragile (the masking tape folded over the top of each sheet helps to prevent cuts!).  Nevertheless, it works!

Deville, E. (1902) “On the use of Wheatstone Stereoscope in Photographic Surveying” Proceedings and Transactions of the Royal Society of Canada Second Series, Vol. VIII:63-69

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The Invention of the Stereoscope

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…

My simple Wheatstone mirror stereoscope.

My simple Wheatstone mirror stereoscope.

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:

A diagram illustrating Leonardo Da Vinci's comments on human sight.

Leonardo Da Vinci commented about what happens when you look at an object. He realised that each eye sees something different.

Try this experiment to see this working:

[1] place a die at the far end of the table;

[2] kneel at the other end of the table and look straight along the table top to the die, with both eyes open;

[3] 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:

Put a die at the far end of a table to understand parallel lines of sight from your eyes.

Put a die at the far end of a table to understand parallel lines of sight from your eyes.

[4] now bring the die to within about 15 cm of your face;

[5] 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.

The diagram of his stereoscope from Wheatstone's paper published in the Transactions of the Royal Society, 1838 (image via wikimedia commons).

The diagram of his stereoscope from Wheatstone’s paper published in the Transactions of the Royal Society, 1838 (image via wikimedia commons).

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.”

My simple Wheatstone mirror stereoscope.

My simple Wheatstone mirror stereoscope.

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”.