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

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”.

The Guilsfield Hoard Sword

The talented Dr H from prehistories kindly joined the North Wiltshire Branch of the Young Archaeologists’ Club in October.   Led by Dr H, the morning’s drawing activities focused on some objects from the group’s handling collection.   The children did a great job and really enjoyed working on comic books and strips to tell stories inspired by the artefacts.  I’ve inked up my effort on the sword from the Guilsfield Hoard.

The Guilsfield Sword

Birch Boat Bowl

My oak and my ash have been giving me nothing but trouble.   Hence no recent progress on the Kingsteignton Idol or the hilt for the bronze sword that belongs to our local Young Archaeologists’ Club handling collection.

But I have finished my first attempt at a boat bowl.   It’s not based on an archaeological artefact.   I was inspired by some of the bowls and the toy boats that I saw in Denmark last year.   Making it has been an exercise in carving – a practice piece.

The silver birch came from a local farm.   There are very few silver birch trees growing where I live, the conditions aren’t favourable for them.   I jumped at the chance to get hold of some logs, because it’s lovely to look at and lovely to carve.   Mostly I get oak, ash and field maple, but the farm has a few silver birches dotted throughout its woodland.

The logs had been in my store for a few months, so the wood isn’t fully seasoned but it’s not sopping-wet either.   However, it has spalted.  The spalting is the patterns of colours brought about by fungi growing in the wood.   If the rot is too far gone then the wood is useless for carving (unless you can impregnate it with an acrylate to hold it together as you cut into it).   If you can catch it just right, however, the wood will hold up and your carved piece has all these amazing patterns and colours in it.

The bowl hasn’t been oiled yet, it needs to dry out fully first.  The oiling should really bring out the effects of the spalting and I can’t wait to see it – so I have to restrain myself and wait until the right time to do it!

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Spalting is one of the ephemeral characteristics of biodegradable materials that is missing from the archaeological record.   However, just as there are beautiful prehistoric axe heads made of multi-coloured and patterned stone, I think people in the past would have enjoyed wooden objects with different colours too.

Hitting things

Sarsen hammerstones

Sarsen hammerstones

Seeing my little bit in Operation Stonehenge  – and being held up with all bar one of my current pieces of work because of problems with raw materials – made me look over the range of the “hitting” tools in my workshop.   So here’s a little photographic review of some hammers, dressers, mallets and mauls.

Raphael Salaman (1975, 1982:218-236) describes 70 tools under the headline “hammer” in his catalogue of woodworking trades’ tools.   Some of these entries describe more than one type of the hammer in question, increasing the number of distinctive hammers beyond 70; such as the entry for the Cooper’s Hammer on page 223.   This includes the London or Burton pattern of hand hammer, the Scotch pattern of hand hammer, the Flue Hammer; and two types of two-handed hammers, the Sledge and the Set.   Salaman describes the  specific characteristics of these five hammers, including their shapes, sizes and uses in coopering.

Salaman records an additional 41 alternative hammer names amongst the tools that he has described in full.   He also has six mallets (pp267-269), one dresser (p380) and there are trade-specific mauls (p270) – see also batter, beater, beetle, cudgel, froe club – for the shipwright, cooper, basket-maker and other greenwood-workers.

Being a record of tools used in woodworking, this book doesn’t include all the hammers belonging to smiths, jewellers, watch- and clock-makers, cobblers, and a myriad other trades.   So there are a lot of hammers out there.

Most of the hitting tools described by Salaman are used for driving.   That is, they knock a thing into, onto, or from, another thing; like a nail into a timber, a wedge into a log, a hoop around barrel staves, a sheet of lead around roof timbers.

In a trade like quarrying there are hitting tools which drive another tool – such as a sledge hammer to drive plug and feather wedges into stone to split it open – and hitting tools which hit the stone directly, to remove material.   These include things like the pick, hammer-axe and walling hammer.   So when is a stone-working tool a hammer, a pick or an axe?

A pick pecks – an axe cuts – a hammer strikes.   These words describe the action, the movement, of the tool at work.   The result of the action depends on a number of different factors, such as the shape of the tool, the force used in the action, the nature of the struck stone, the angle of the strike, the position of the point of impact.

The pecking stones, for example, which are made from nodules of flint, remove tiny grains from a stone surface.   I am using them to pick away at a piece of sarsen stone to make an even surface finish.   The knapping hammers, however, remove whole flakes – from large flakes which themselves might be knapped into other tools, to tiny little retouching flakes to finish a tool edge.  Here’s a short video of Karl Lee describing some of his knapping hammers.

The knapping hammers have characteristic damage at certain places which show where they hit the flint and how they are held to do it.   They are more like the hammers in the top group of photos, which also have one or two main surfaces that come into contact with the thing being hit, and which are held in a particular way.   In contrast, the pecking stones can be held any-which-way.   They have many angular, sharp edges all over that abrade the surface they hit.   Eventually these will all wear down and the stones will become useless.

All these different types of hitting tools…how can an archaeologist usefully talk about them?

In 1901, William Gowland carried out a small excavation at Stonehenge prior to straightening one of the huge sarsen stones.   Stone 56 looked like it was going to fall over.   Other sarsens had come crashing to the ground and broken into pieces in the past; but stone 56 is really handsome and the only part of the Great Trilithon still standing.   No one wanted to see it come to a sad end.   Here’s a pair of before and after photographs on Timothy Daw’s blog to show you what Gowland did.

Gowland was the first person to identify and classify tools used on Stonehenge’s stones.   During his excavation he found a range of beaten-up nodules and rocks which he interpreted as the tools used to shape and dress the standing stones.   Using a sample of 100, he divided them into five groups by material and weight.   He called the smallest groups axes and hammerstones; the middle group weighing from 1lb to 6½lb, hammerstones; and the largest, weighing more than 36lb, mauls (Gowland 1902:62).   He then described the different tasks he thought the tools would have been used for, such as the biggest mauls for knocking off lumps of stone to make the rectangular shapes of the standing sarsens.

This is a bit like the difference between a lump hammer and a sledge hammer.   One is a larger version of the other, and because of the sledge hammer’s greater weight, greater striking area and longer handle, it can do heavier work than the lump hammer.

On the other hand, you could think about classifying the lump and sledge hammer in terms of the action used to wield them (one-handed, two-handed…); the people that use them (brick-layer, navvy, convict…); the shape of the head (four-sided, eight-sided…); the shape of the handle (short, long, straight, curved…); the other tools each is used with (chisel, pick, crow-bar…) – and probably a hundred and one other ways, including the manufacturer, the forging technique, the source and quality of the metal…

Since the nineteenth-century, archaeologists have been grouping and dividing classes of object into types – typologies – with the aim of putting the things into relative chronological order.   Working out the age of something is a really important question to answer.   Working out what something was used for, and who used it, are just as important –  answering different questions like these might require the archaeologist to group the same objects in different ways.   It all depends on what you want to find out, like Salaman grouping together the hammers of many different trades, or Gowland dividing tools for one specific job (preparing bits of Stonehenge).

However you group or divide the hammers depends on how like or unlike one is to another.   Are my mauls more like my lead dressers, because they are each made of one piece of wood?   Or are my mauls more like my lump hammer and railway track hammer, because they are similar weights?  Are my pecking stones more like my sarsen hammerstones, because they are made of a silicified type of stone?   Or are my knapping hammers more like my mason’s axe because they chip bits of stone away?   Should I just lump them all together as “hammers”?   Or are they all so different that I should split them up into sixteen different types of hammer?

Archaeology has numerous techniques to cluster objects together or to divide and sub-divide them into ever smaller groups.   It all depends on what you are interested to find out – and whether you are a —

...lumper or a splitter.

…lumper or a splitter.

Gowland, W. (1902) “Recent Excavations at Stonehenge”  Archaeologia 58(1): 37-118

Salamon, (1975, 1982) Dictionary of Tools Used in the Woodworking and Allied Trades, c.1700-1970   London: George Allen and Unwin

Operation Stonehenge episode 2

The forecast for the December day made it look cold and grey outside, even though it was still dark at 6am.   In the brightly-lit farm kitchen the sizzling bacon smelt fabulous.   We all tucked in – bacon rolls, bowls of cereal, mugs of coffee and tea.

As daylight broke, we drove up towards the Down.   In Pickledean, the cameraman set up gear including a crane for overhead shots and a bed for controlled panning at grass level, while the sound guy complained about the rustling made by synthetic fibres of modern outdoors clothing.



Much ambling about the stones later, and talking, and filming, and more talking, and it was time for lunch.

In the afternoon, the sky grew ever more grey as the sarsen slowly grew more white.  This is what they had really come for.   Action shots; noisy shots; things that look good on telly.   Technology; experience; knowledge; clever prehistoric people who did amazing things with simple materials.   Other things that might surprise the general viewer – who knew that bits of Stonehenge were once gleaming white?*

Worked sarsen

Worked sarsen

A day’s work for a few minutes of a TV programme.   Possibly the hardest thing I’ve ever done yet.   It’s easy to extemporize on a subject; less easy to stick to a story line and give the Director exactly the words and style of delivery he’s after with only a few minutes of instruction.   I think I’d do it much better if I was asked again.

You’ll be able to watch it via the BBC iplayer for a bit, and no doubt there’ll be repeats.   Operation Stonehenge, episode 2, was broadcast on BBC2 on 18 and 20 September.

* all of the Stonehenge sarsens, according to the Production team, rather than just the ones that were worked, and the ones that didn’t have too much brown iron oxide running through them.

Drawing the Kingsteignton Idol

Back from Spoonfest, and internet access restored!

Before I went away I had been working on an important stage in making a replica of the Kingsteington Idol.   Drawings.

They aren’t my drawings.   This isn’t a commission.   I haven’t arranged to visit the Royal Albert Memorial Museum in Exeter to draw and photograph the Iron Age wooden figure.   It’s fragile and very special, best only handled when really necessary.   The drawings are those published in Bryony Coles’ (1990) paper about prehistoric carved figures.   A colleague very kindly scaled them up for me to save some time.

Relying on the published drawings for the form and dimensions of the figure presents problems though.   The front, back and one side of the figure only are illustrated.   Measurements can be taken from these perpendicular views, providing maximum and minimum but it is much harder to capture intermediate measurements without being able to examine the detail of the changes of each surface.

Nevertheless, I have traced the outlines and cut out templates, reversing the single side view to make its pair.   To see how these shape up, I have worked up a section of the Idol in a small piece of sweet chestnut.   Sweet chestnut can behave like oak (oak is what the Idol is made of) because it has similar properties.

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Not too shabby, but more work required!

Coles, B. (1990)   “Anthropomorphic Wooden Figures from Britain and Ireland”   Proceedings of the Prehistoric Society 56:315-333