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”.

Weird and wonderful #3

Not weird but certainly wonderful, I’ve been handling a good deal of ash and oak recently.   These have been firewood logs, however, with only a few bits suitable to be set aside for carving and other purposes.

Restrop Farm, where all my fuel comes from, is a small farm of rolling pasture fields and woodland.   Included within its bounds is Ringsbury Camp, an Iron Age hillfort (scheduled monument 1018124) which sits on a spur of rhaxella chert looking out from the limestone ridge over the head of the Thames Valley.   The views to the north, towards the Cotswold Hills, are fabulous.

About two-thirds of the hillfort banks and ditch are covered in trees, which along with the burrowing badgers have been compromising the integrity of the earthworks.   The interior has been ploughed in the past, although only for a relatively small part of the twentieth-century after the Second World War, and not (as far as I know) with deep ploughing equipment.   For many years the farm was owned by three sisters, the Misses Paginton, and their dairy farming practices were old-fashioned and gentle.   It is to be expected that archaeological remains inside the hillfort are in good condition, but aside from a few stray finds there has been no controlled excavation here.

Sometimes the yellow stony scree thrown up by the badgers, which rolls down the banks onto the hillfort’s encircling paths, is a reddish-brown colour.   This is the bloodstone, reputed in local story to be the stone stained red with the blood of unknown warriors who died in combat here – an event commemorated by the name of the nearby lane “Battlewell”, and “Red Street”, the alternative name for this part of the parish.

Ethel Richardson, in her local history published in 1919, mentions the possibility of there having been a Civil War skirmish here, when the lanes ran red with blood.   She ties the red of battle to the red which at the time was thought to be the origin of the place-name Restrop.   The historical truth is in a way more prosaic, and yet (to me) more romantic.    Restrop is derived from “Rada’s thorp” (Gover et al 1939); Rada, the name of the otherwise invisible Anglo-Saxon farmer who cared for his crops and animals on the hillside and in the coombes which cut through the limestone, looking out over the clayey Thames valley to see wisps of smoke rising from the hearths in the walled town of Cricklade to the north.

Oak and ash both grow well at Restrop, and useful woods they are.   They are both ring-porous.   The vessels which transport water and minerals up from the roots to the tree leaves are made up of cells that make long tubes.   Those vessels formed during rapid Spring growth are much larger than the tubes that grow in the Summer, thus forming rings which emphasise the tree’s growth rings (Abbott 1989, 2007:17).   You can see this in the photo below:

Ash and oak

Ash and oak

The piece of ash on the left has especially clear growth rings.   The rings of vessels are the darker, brown-coloured lines, in-between the pale wood fibres.   This piece comes from a faster-growing branch, which you can tell because there are only four or five growth rings to the inch.   Faster-grown ash is stronger than slow-grown because it contains more wood fibres, which is good for shock-resistance and is why ash is good for tool handles and furniture.   Ash grows best on deep soils over limestone; just like the banks of Ringsbury Camp.

On the right, the oak – showing the very clear distinction between the dark heartwood and the light sapwood.   Oak has a very strong and rich smell, owing to its acidic tannin content.   This makes the wood resistant to rot (but also eats into steel so I am very careful to clean my tools quickly and thoroughly after using them on oak) (Abbott 1989, 2007:26, 28).   There are a number of big oaks at Restrop, which have built themselves for strength against the wind that blows up and down the limestone ridge, and the prevailing weather rolling in down the valley from the west.   Oak is a sinuous wood.   Its strength and longevity have resulted in more than 90% of building timbers being taken from oak trees (Rackham 1986, 1995:86).

Now that the hedgerows and woods are greening up, it’s time to take a good look at the ash and oak growing at Restrop to see which buds come out first,

“Ash before oak – in for a soak;

Oak before ash – in for a splash.”


Abbott, M. (1989, 2007)  Green Woodwork   Lewes: Guild of Master Craftsman Publications

Gover, J.E.B., Mawker, A., Stenton, F.M. (1939)  The Place-names of Wiltshire   English Place-Names Society/Cambridge University Press

Rackham, O. (1986, 1995)  The History of the Countryside   London: Weidenfeld and Nicholson

Richardson, E.M. (1919)  The Story of Purton; a collection of notes and hearsay   Bristol: J.W. Arrowsmith Ltd

This post is the third in an occasional series called “Weird and Wonderful”.

Weird and Wonderful #2

At last, some good weather!   Finally, I have cut my willow.   I wouldn’t normally have left it this late, but it’s been so difficult to get on the ground in this rain.   Neither do I have the right space to store too much greenwood; so it’s really important that my raw materials are in tip-top condition to keep, if I’m not going to use them right away.

These considerations bring to mind two issues: good materials; and seasonality.

I grow different varieties of willow, each of which is a basketry type: Salix purpurea “Dicky Meadows”; Salix purpurea “Lancashire Dicks”;  Salix triandra “French”; Salix daphnoides “Continental Purple” and Salix alba vitelina “Britzensis”.   Here is the first wonderful thing about willow.   Many varieties produce great rods for basketry – good for their long, slender, straight rods with strong skin, and with flat, close-growing buds.

In Britain there are eighteen native willow species and twenty-seven interspecific hybrids (Brendell 1985:2).   The second wonderful thing about willow is that it is fabulously successful at reproducing.   There is widespread hybridisation, it flowers and seeds, and it will “strike”; push a 9″ stick of willow into the ground in early Spring, and it will root and grow for you.

Following on from willow’s strong inclination to expand its tribe is its determination to keep on growing.   Cut back some willow, and return next year to see how well it has come on!   Be amazed at all the straight rods growing out of the treetop where a branch has fallen away in a storm!   I imagine that it would take no more than a season to realise how easy, and how valuable, it is to manage willow to grow really useful products; from basketry rods to fence- and building-sized poles on pollarded trees.

Which brings me to seasonality.   It’s time to cut the basketry rods once all the leaves have fallen off.   This is usually from November.   The rods are then a season old, good and straight with no branching, the buds are small and flat, the leaves have fallen naturally.   You can continue to cut over the Winter, until the sap has started to rise again, when the buds on the rods will start to push on.   As long as you can keep your harvested rods cool and dry (beware mould and frost), you don’t have to use them right away.   They change from “greens” = unseasoned rods, to “browns” = seasoned rods.   All it takes is the right amount of soaking and damping when you need them, and you can make baskets with your browns any time of the year.

So if you do want to store your rods, this has an impact on the building space that you need.   The right space for storage, the right space for soaking and mellowing, the right space for working (although working space is the least demanding).   Which is another way of saying that it was not necessary in the past for life to be entirely governed by the seasons, given planning and preparation.

Last year, however, the lack of frosts meant that my plants still had leaves well into December.   Then it seemed that it would never stop raining.   If I had cut my rods a few weeks ago, I would have risked loosing the lot to rot in storage.   As it is, I was only just in time, as you’ll see from the photos (the buds, especially towards the tips, were starting to sprout).   The photos also suggest another wonderful thing about willow, the beautiful range of bark colours.

What the pictures can’t convey, however, is the last wonderful characteristic of willow that I want to highlight.   The fabulously spicy, salty, aromatic smell.

Willow harvest, March 2014

Willow harvest, March 2014

Willow harvest, March 2014

Willow harvest, March 2014

Brendell, T. (1985)   Willows of the British Isles   Princes Risborough: Shire Publications Ltd

This post is the second in an occasional series called “Weird and Wonderful”.

Weird and Wonderful #1

Last week I attended EAC2014 – the eighth annual Experimental Archaeology Conference – held this year in Oxford.   More on that later, I shall write a post about my experience of this interesting, informative and fun gathering shortly.

Visiting Oxford offers the opportunity to visit the fabulous Pitt Rivers Museum, and the Conference organisers had arranged sessions specially for participants to experience the incredible world of Henry Augustus Lane-Fox Pitt-Rivers and all the collecting that’s gone on since that eminent Victorian gathered in the world’s things.   The museum describes itself as “the University of Oxford’s collection of anthropology and world archaeology”; the understatement does it no justice.   It’s a surprising, shocking, life-affirming treasure-trove of human ingenuity.

There is so much to take in.   The objects are famously displayed in their Victorian fashion, with uninterpreted, crammed cases devoted to types of thing rather than place/country/period/culture.   Fascinating and frustrating in equal parts, this antiquarianism is a very important part of the museum’s character and delight.

It also makes it easy to see things for the first time, no matter how many visits you’ve already made.   This happened to me on Friday last week.   Just beside the side-door through which we entered the museum, I spotted this (forgive the poor photo, I took it using my mobile, in the low lighting):

A porcupine fish skin helmet from Kiribati (Gilbert Islands).

A porcupine fish skin helmet from Kiribati (Gilbert Islands).

It’s object number 1884.32.31, collected before 1878 having been purchased at auction.  It traveled around various of the London museums when in 1884 it was donated, one of the Pitt-Rivers “founding collection” objects.

It’s a bizarre object, part of a set of body armour designed to protect the wearer from weapons edged with shark’s teeth.   The helmet’s arresting appearance would have served to make the wearer look as threatening as possible.   Who on earth ever thought of skinning a fish to make a helmet?!   What processes were required to cure the skin and ensure that the head-sized shape was kept open until the skin stiffened?   And how was all this done while the poison tips of the spines were still active?

I admit that my perception is probably strongly influenced by two key issues.   First, British native fishes don’t naturally spring to mind as potential sources of defensive clothing.   Secondly, there isn’t anything quite like this in the British archaeological or ethnographic records.   It’s easy to be amazed and puzzled by the exotic.   The key thing to remember is that, “Kiribati is comprised of low-lying coral atolls, and so very few raw materials are available. Consequently, this armour is made from woven coconut fibre and fish skin.” (Mills 2006-7).

The porcupine fish skin helmet may be weird and wonderful, but it serves to remind us of the importance to people’s lives of access to materials, ingenuity of thought and skill in making.

Mills, A. (2006-7)  Notes, Pitt Rivers Museum Catalogue entry 1884.32.31 [online database] [accessed 15-01-2014]

This post is the first in an occasional series called “Weird and Wonderful”.