Zograscope

A zograscope is an optical device that has a lens that makes a picture placed behind the lens appear very large and far away, giving the perception of being immersed in an environment. It was a parlor entertainment of the 18th and 19th centuries, used to view perspective prints known as vues d'optique.

The picture is placed at about the focal distance of the lens, so that light from each point on it reaches the eyes almost parallel, as if it came from a much more distant scene.

It can have a mirror at an angle so the image can be placed flat on a table.

Unlike the later stereoscope, the zograscope shows one single picture, not a stereo pair, and both eyes see the same image. Vision scientists Jan Koenderink, Maarten Wijntjes and Andrea van Doorn found in 2013 that it works by removing the physiological cues that reveal a picture to be flat, which lets the purely pictorial depth cues dominate. Art historian Erin C. Blake has described the experience of 18th-century zograscope users as an early form of what is now called virtual reality, and Stephen R. Wilk, writing in Optics & Photonics News, discusses it as a form of 3-D viewing that predates stereoscopic imaging.

Reviewed 27 September 2026. Checked every claim against Koenderink et al. 2013 (i-Perception), Blake's 2003 MIT Press chapter, Wilk's 2012 OPN article, the Yale, Cleveland, Princeton, Bantjes, World Wide Words and LaValle pages, and the Chaldecott DOI record. About review dates.

Construction and use

According to Blake, the essential part of a zograscope is a convex lens at least three inches in diameter, with a focal distance of about an arm's length. In the 18th century the lens was usually hung vertically in front of a mirror inclined at 45 degrees, and the assembly was either mounted on a stand or built into a box that could stand on a parlor table. The viewer looked with both eyes through the lens at the reflection of a print lying flat below it. Because the mirror reverses the image from left to right, many prints made for the device carried their titles in mirror-reversed lettering so that they read correctly when seen through it.

Surviving examples vary in size and finish. The Yale Center for British Art holds a late 18th-century English zograscope of mahogany, glass and mirror, 64 cm high on a 21 cm base, whose catalog entry describes it as a wooden stand supporting a hinged mirror and lens. A zograscope in the collection of Rod Bantjes of St. Francis Xavier University has a biconvex lens 12 cm in diameter with a focal length of 152 cm. Some American examples were built into furniture. Stephen R. Wilk describes a five-foot painted wooden obelisk at the Museum of Fine Arts, Boston, with a lens about eight inches in diameter behind a carved panel attributed to Samuel McIntire of Salem, and a 45 degree mirror at the top that turns the vertical view of the print into a horizontal one. Edward Johnson of Salem built at least two desks with a built-in viewer between 1793 and 1811; one is at the Peabody Essex Museum and the other at the Winterthur Museum. Wilk also records cheaper folding models that were little more than a frame, lens and mirror, and itinerant showmen who carried a zograscope and pictures from place to place and charged a fee to look.

In more elaborate versions the prints had cut-outs covered with colored tissue and could be lit from behind to give "night" views of cities.

How it works

Koenderink, Wijntjes and van Doorn published the first detailed optical analysis of the device in 2013, noting that such an analysis was not available in the literature and that common explanations of the apparatus were "evidently nonsensical". In their account, the observer views the picture through a large positive lens with both eyes, the picture lying in the focal plane of the lens. The field of view is about 30 to 40 degrees, so the observer explores the picture with eye movements. Because the picture is at the focal plane, the observer does not have to change vergence or accommodation when looking from one point to another, and the two eyes receive identical images. This removes the physiological cues, including binocular disparity, vergence, movement parallax, accommodation and blur, that would otherwise show the picture to be a flat surface. The result is what the authors call "monocular stereopsis": a vivid impression of increased "plasticity" produced by the pictorial cues alone, similar to looking at a large painting from a distance with one eye closed.

Blake gives a related description: light from the print arrives at the eyes nearly parallel, and spherical aberration near the edges of the lens, together with juxtaposed patches of bright color on the print, further disrupts normal depth cues, so the scene stretches into depth while the individual figures, carriages and trees remain flat but appear spaced apart. Wilk notes that the large single lens, with its aberrations, gives the impression of looking at a spherical surface that rotates as the viewer moves, and that the image was not stereoscopic in the modern sense.

A single lens with an aperture of about f/3 to f/4 should, by conventional optical design standards, show large spherical, astigmatic, comatic and chromatic aberrations. Koenderink and colleagues argue that this reasoning does not apply because the beams are limited by the observer's pupils, about 4 mm across, and not by the lens rim, so the system has to be analyzed as two narrow beams separated by the interocular distance. Small, unconscious vergence and accommodation adjustments then correct field curvature without acting as depth cues. Ray tracing a 120 mm plano-convex lens of BK7 glass (radius of curvature 120 mm, effective focal length 271 mm, interocular distance 65 mm, 4 mm pupils) and a picture 200 mm wide, they found blur of no more than a quarter of a millimeter at the far corners of the image and under a tenth of a millimeter over most of it. They identified chromatic aberration as the main flaw, since lateral color is mirror-reversed between the two eyes and produces a chromatic disparity signal, though they reported that it is hardly noticeable in structured or monochrome images. The best lens form in their simulations was plano-convex with the flat side toward the observer.

The authors also note that some zograscopes use a concave mirror in place of the lens, which removes chromatic aberration, but that they had only seen such mirror devices for pictures smaller than postcard format. They suggested that a modern zograscope could use a plastic Fresnel lens.

History

Lens viewing before the zograscope

The depth effect of viewing pictures through a convex lens was described before zograscopes were sold. Blake cites the German writer Johann Christoph Kohlhans, who in 1677 described how fitting a camera obscura with a convex lens made a pictured object appear as if before the naked eye, and William Molyneux of Dublin, who wrote in 1692 that "Pieces of Perspective" appear "very Natural and strong through Convex Glasses duly apply'd". According to Blake, instrument sellers and print sellers of the period did not yet market lenses for picture viewing. In 1740 Benjamin Martin described the effect of such viewing on a portrait: "The Cheeks are protuberant, the Nose projects, the Eyebrows overhang . . . and the Lips are about to speak".

The English craze of the 1740s and 1750s

Blake describes the zograscope as the center of the first 3-D gadget craze, which developed in England in the 1740s when convex lenses and perspective prints began to be sold together as consumer goods. She dates the earliest known mention of prints for use with the device to the St. James's Evening Post of 2 to 4 April 1747. Newspaper advertisements peaked in 1749, the year The Gentleman's Magazine described zograscope viewing, and in 1750 and 1751 the magazine included four views for the device in each annual volume. By 1753 a separate catalog listed more than two hundred zograscope prints in the stock of Robert Sayer. Blake counts 285 zograscope prints advertised in London newspapers between 1747 and 1753, after which the number of new views fell sharply.

Contemporary users stressed how lifelike the views were. Blake records that William Emerson praised the device for putting viewers inside a summer scene even on a winter day, and that Henrietta Knight, Lady Luxborough, wrote in 1748 that she preferred seeing London through her zograscope to visiting the city. By the 1760s zograscopes were still sold in large numbers in England but had become an ordinary amusement, and production of zograscope views developed in France, Germany and Italy.

Later use

Wilk lists London, Paris, Bassano in Italy, Augsburg in Germany, Amsterdam and Vienna as production centers for the viewers and their prints. He notes that the devices were favored in Europe but relatively rare in the Americas; the Boston obelisk once belonged to Samuel Holten of Danvers, and Joseph Warren Revere, a son of Paul Revere, bought a similar device in London in 1800. A 1795 catalog quoted by the Yale Center for British Art offered "Diagonal mirrors, neatly fitted up in mahogany, with looking-glass, &c. One guinea each." Bantjes reproduces an illustration of the device from C. F. Casella's 1871 catalog of scientific instruments and records the name "optical diagonal mirror" from that year.

Koenderink and colleagues write that the zograscope was in common use to at least the end of the 19th century, that it was popular with the well-to-do much as the stereoscope was in Victorian times, and that combined stereoscope-zograscopes were made around the turn of the 20th century. Its design did not change after its invention in the 18th century. In the early 20th century Moritz von Rohr, an optical engineer at Zeiss, developed "synopters" that produced the same effect with much better optical quality.

Names

The device had several names. Eighteenth-century advertisements quoted by Blake use names such as "Diagonal Mirrour" and "Optic Machine", and Blake notes that "zograscope" is the name commonly used today. Other names include optical diagonal machine and optical pillar machine. In 1953 J. A. Chaldecott published a study of the instrument under the title "The zograscope or optical diagonal machine". The origin of the word is uncertain. Koenderink and colleagues and the lexicographer Michael Quinion both describe it as unclear. Bantjes derives it from "zographie", the term that the mathematician John Dee used in 1570 for the rules of drawing in perspective, which would make a zograscope a machine for viewing perspective to best advantage.

Vues d'optique

The prints made for the zograscope, called vues d'optique or perspective views, were designed to strengthen the sense of depth and usually showed cities, notable sites or events. In England almost all of them depicted the built environment. About half of the prints in John Bowles and Son's 1753 catalog show urban scenes such as busy streets and rivers, groups of buildings and cathedral interiors, and most of the rest show formal gardens. The standard English zograscope view measured 11 by 17 inches, in landscape format, with the title in the lower margin where it would not interfere with the illusion. A catalog of 1784 quoted by Quinion offered "remarkable Views of Shipping, eminent Cities, Towns, Royal Palaces, Noblemen and Gentleman's Seats and Gardens" at a shilling plain or two shillings colored.

Blake notes that viewers in England do not seem to have minded that the mirror reversed the scene and caption, and that deliberately reversed prints with reversed captions were produced in large numbers only somewhat later in France, Germany, Italy and elsewhere. Julie Mellby of Princeton University Library writes that the Augsburg publisher Georg Balthasar Probst ran a busy studio producing prints labeled in four languages for sale throughout Europe, and that the 56 hand-colored perspective prints given with a zograscope to Princeton's Graphic Arts Collection include views of the major cities of Europe as well as Mexico, Egypt and the Far East. The Cleveland Museum of Art holds The Optical Viewer (about 1793), an etching by Frédéric Cazenave after Louis Léopold Boilly that shows a fashionable woman and a boy looking at prints through such a device.

Relation to virtual reality

Blake titled her 2003 study of the device "Zograscopes, Virtual Reality, and the Mapping of Polite Society in Eighteenth-Century England". She argues that the descriptions by Emerson and Lady Luxborough describe the visual side of "what is today called 'virtual reality,' a space that is enacted technologically and perceived by viewers but that exists nowhere on its own", and that, like the virtual image of the nineteenth-century stereoscope, the zograscope view has no original elsewhere and exists only while viewer and device come together. Wilk writes that zograscopes are an early demonstration of how depth cues in images can be enhanced, by minimizing the signals for flatness and making the visual angle close to that of the original scene.

Not all writers accept the immersion framing. Bantjes points out that when the viewer stands back far enough from the lens for the edges of the print to be hidden (about 18 cm), the image fills only a small part of the visual field, and the viewer sees the whole mechanism and more of the room beyond than of the pictured scene. He argues that the open structure, which exposes the lens and mirror, and the reversed captions that invite reading while viewing, suited 18th-century "rational recreation" and encouraged a critical distance on the part of the user.

Modern head-mounted displays also view a picture through a magnifying lens placed at about its focal distance. In his textbook on virtual reality, Steven M. LaValle explains that VR headsets use a convex lens as a magnifier, with the screen placed at about the focal distance so that it appears as an enormous virtual image that is infinitely far away, the same situation as in the View-Master. The zograscope differs in using one large lens shared by both eyes to view a single picture, whereas a VR headset places a separate lens in front of each eye.

Research

To test whether the zograscope works as its users claimed, Koenderink, Wijntjes and van Doorn built one from a salvaged plano-convex epidiascope lens 12 cm in diameter with an effective focal length of 380 mm, using an iPad (211 by 159 mm, 2048 by 1536 pixels) as the picture and omitting the usual diagonal mirror. The three authors served as observers and used a gauge-figure method to map the perceived relief in two photographs of sculptures and a painting by Amedeo Modigliani, with and without the lens. For the two sculpture photographs, relief depth was greater with the zograscope for every observer, with magnifications ranging from 4 percent to almost a factor of three depending on the picture and the observer:

Picture	Observer AD	Observer JK	Observer MW
Sleeping Hermaphroditus (photograph of sculpture)	1.42	1.88	1.04
Danaid, Auguste Rodin (photograph of sculpture)	1.4	2.96	1.36
Reclining nude, Amedeo Modigliani (painting)	0.72	0.72	1.08

The values are the slopes of the regression of depth with the zograscope against depth without it. The Modigliani results correlated poorly between conditions; a cluster analysis split that image into two parts whose magnifications were above 1 for every observer except one cluster for observer AD (0.94). The authors attribute the smaller effect to Modigliani's painting style, which preserves awareness of the flat picture plane. They concluded that the zograscope is a form of synoptic viewing that magnifies pictorial relief in a way that depends on both the picture and the observer, so it cannot be given a single "depth magnification" figure.

See also

• History of virtual reality
• Stereoscope
• View-Master
• Depth perception
• Stereopsis
• Virtual image