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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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

About this blog

Image

Here are the bowls of two wooden spoons that I made a while back.   They are carved from sweet chestnut.   You can see the front of the bowl of the smaller, heart-shaped spoon and the back of the larger, shield-shaped spoon.

This blog is mostly about objects that I have made.

There are some really interesting blogs written by amazing craftspeople about their products, specialist skills and experiences.   If you want to learn about one specific craft or technology, you will need to find one of those blogs.

This blog is all about a range of objects, material, tools, technology and processes.   You will find that these usually have something to do with the archaeological record, archaeological theory or archaeological interpretation.

If you look carefully at the blog header image, you will recognise that it is a photo of the heart of an open fire that is being used to turn clay pots into ceramic – the firing process which in Britain has helped people from the Neolithic period onwards to live their lives.   The photo is one of a sequence that documents a whole firing process.

Objects are made by people in places, and often this too will be reflected in my blog posts.

The sweet chestnut logs from which I cleft the billets that I carved into the spoons came from the grounds of a primary school.   The elderly tree was causing problems to the school buildings and was also unwell, so it had to be taken down.   Having selected a few lengths, I brought the logs home with some help and put them into storage.  Later on, I started to reduce the large logs into sections for carving different objects in my workshop.

I might have done things differently if I had been carving spoons in the Bronze Age, say.   Interpreting the past is a knotty and contentious practice, as some of my objects will show you.