Stereoscopy: Charles Wheatstone's phenomenon of binocular vision

Applications of stereoscopy include 3-D movies, immersive gaming and virtual reality simulators. All of which work on the same basic principal of viewing dual images, one with each eye.

These technologies could not have been developed without an understanding of the physiology of human vision.

Stereoscopic trickery is possible because humans have binocular vision – two eyed vision. Our brain is designed to process two images, one from each eye, seeing slightly different views from their side by side placement on our face. The visual cortex rationalises the ‘binocular disparities’ in the images from each eye to create a cohesive view of the world with accurate perception of location and depth.

Since its foundation in 1660, Fellows of the Royal Society had presented new discoveries and inventions to their peers at meetings. These papers often went on to be published in the Society's Journal where they could reach a wider audience. Wheatstone's paper on stereoscopy: 'Contributions to the physiology of vision. Part the first. On some remarkable, and hitherto unobserved, phenomena of binocular vision', appeared in The Philosophical Transactions of the Royal Society in 1838.

In his seminal paper Wheatstone first establishes that each eye has a different field of vision, and so the brain must be receiving different images from each retina - binocular vision. According to Wheatstone this observation had only previously been noted in writing by Leonardo Da Vinci and never fully explored. He goes on to outline experiments for forcing each eye to look at separate objects or images and observing the resulting phenomena, to prove stereopsis or depth perception is a phenomenon of binocular vision. A contemporary who reviewed the paper noted they were 'ingenious experiments of valuable observations'. Some of these experiments employed Wheatstone's newly invented reflecting stereoscope.

Wheatstone had trained his eyes to see properly prepared pairs of images or stereograms in three-dimensions without a viewing aid. He designed the reflecting stereoscope to help those with less practiced eyes. His paper provides illustrated instructions on the use of the stereoscope to view flat images in relief - an entirely new 3-D optical experience.

The first stereograms, created by Wheatstone were pairs of basic line drawings published alongside the design of his stereoscope. These were deliberately simple as the focus of the Royal Society paper, presented to his scientific peers, was on demonstrating his observations on optics rather than providing entertainment. Wheatstone thought complex, shaded drawings might cause users of the stereoscope to think the 3-D effect was an artistic illusion rather than the result of using the stereoscope.

In a second paper on the physiology of vision in 1851, Wheatstone recognised that it was impossible for even the most talented artist to create the necessary projections for stereograms that would accurately reproduce complex objects and scenes. Whereas two stereoscopic photos with the desired perspective could be relatively quickly and easily produced regardless of the intricacy of the subject.

To produce an effective stereograph each image must be taken from a different position on the same horizontal axis. The position depends on the distance of the subject from the camera lens and the distance of the images from the mirror or lens of the stereoscope used to view them.

Early examples of photographs by Henry Fox-Talbot. Many others were working on varying photographic processes in the same period. Find out more in the Royal Society's online exhibit The invention of photography

Although many of Lady Rosse's photographs were taken as stereo pairs, the layout prepared for publication in 1861 (below, left) included only single images. It was not yet possible to print in greyscale or mass reproduce photographs, so the images which appeared in the final publication (below, right) are engravings made by the artist James Basire.

The economics of printing a scientific journal and the difficulty for an artist to accurately reproduce the binocular differences meant it was not practical to reproduce the stereoscopic images as photographic prints or engravings.

Charles Piazzi-Smyth began his career as an assistant astronomer at Cape Observatory, South Africa in 1847 where world-renowned astronomer Sir John Herschel FRS (1792-1871) was in residence. Herschel was also a pioneer of photography, developing the cyanotype process, amongst others. It seems Herschel taught PIazzi-Smyth some photographic skills, later endorsing Piazzi-Smyth's plan to photograph his expedition to Tenerife and calling him an expert photographer.

In 1856, now Astronomer Royal for Scotland, Piazzi Smyth was accompanied to Tenerife by his new wife Jessica 'Jessie' Duncan Piazzi Smyth (1812-1896). Above she is seen in stereo at their camp on Mount Guajara with a mounted 3.6 inch Sheepshanks telescope. Jessie was a geologist and, like her husband, a photographer.

Their photographs documented their experiments and equipment in the field, but also geological formations in the volcanic mountains, Tenerife flora and landmarks.

A special book-stereoscope could be purchased to view the stereographs within the volume.

The motivation for this experiment is unclear, though a projectile makes for dramatic stereoscopic subject, it may have been conceived to help with McKinlay's professional duties. As Proofmaster, McKinlay's role was to test the safety of gun barrels by using an over-pressure charge of powder to strain the barrel's resilience. In artillery proving is the only use for galvanic tubes, which may have given him the idea for the trigger system in his experimental photography.

One David Brewster FRS (1781-1868), researcher in optics and creator of the kaleidoscope enters the history of the stereoscope with a legitimate claim as the inventor of the first portable 3-D viewing device and a stereo camera.

First made in 1849 Brewster's lenticular stereoscope used prisms rather than mirrors. It was a more compact device than Wheatstone’s reflecting stereoscope and the eyepieces also made it easier to use, it became the first commercial stereoscope and by 1851 had gained mass popularity. Many other stereoscope designs went on to be patented over the years. Such is the natural progression of science, innovators building on the inventions of previous generations to re-imagine and improve them.

Brewster, however, was also an antagonist in this tale, claiming that Wheatstone had no interest in the use of photographs with the stereoscope and that this was his own innovation. Brewster even went so far as to suggest someone else had a prior claim to the whole concept of the stereoscope, showing little spirit of fellowship amongst these Fellows of the Royal Society.

Wheatstone launched a thinly veiled defense in his second paper on binocular vision in 1851. He acknowledged the innovations of Brewster but asserted that his reflecting stereoscope was superior in functionality and revealed a more portable version that folded into a case.

In a twist to the tale Brewster's wider work on optics, included defining the laws of polarisation. Nearly a century later polarisation would be pivotal to the technology behind moving 3-D images developed by Edwin Land FRS. Land's 3-D film projected two prints onto the cinema screen in synchrony, the images were then separated by polarised light-filtering glasses worn by the audience to force the binocular effect. It was first demonstrated in New York in 1936.