Stereograms mark a threshold in understanding visual perception. Modern study of stereopsis began with Wheatstone&#x27;s invention of the stereogram and stereoscope (~ 1832), important tools in vision research and technical imagery ever since. Stereoscopic images formed with frieze and wallpaper patterns in illuminated Insular manuscripts such as the Book of Durrow (~ 680 CE), Lindisfarne Gospels (~ 700-720), and Book of Kells (~ 800) show that, long before spectacle-quality magnifying lenses (~ 1286), illuminators somehow copied multicolored, microscopically detailed designs _freehand_ with an accuracy unsurpassed in scientific instruments until the Renaissance (but well within the power of normally sighted humans&#x27; stereoscopic discrimination). If the artists accomplished this feat by free fusion using the unaided eyes as a magnifying stereocomparator, as suggested, they should have been able to create autostereograms. Did they? Here I report two examples of an apparent stereopair from the Book of Durrow, which gives a sharp, strongly three-dimensional image that includes, among other symbols, an eye-shaped sign identified with mirages (Fig. 1). Apparently, precocious empirical knowledge of stereopsis played more than a technical role in the creation of some of the world&#x27;s more famous graphic art

John L. Cisne

English

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An apparent medieval stereogram incorporating a symbol for optical illusion   
John L. Cisne 
Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York  14853 
Stereograms mark a threshold in understanding visual perception.  Modern study of stereopsis 
began with Wheatstone’s invention of the stereogram and stereoscope (~1832)1, important tools 
in vision research and technical imagery ever since2,3.  Stereoscopic images formed with frieze 
and wallpaper patterns in illuminated Insular manuscripts4 such as the Book of Durrow (~680 
CE)5, Lindisfarne Gospels (~700-720), and Book of Kells (~800) show that, long before 
spectacle-quality magnifying lenses (~1286), illuminators somehow copied multicolored, 
microscopically detailed designs freehand with an accuracy unsurpassed in scientific instruments 
until the Renaissance (but well within the power of normally sighted humans’ stereoscopic 
discrimination)6.  If the artists accomplished this feat by free fusion using the unaided eyes as a 
magnifying stereocomparator, as suggested6, they should have been able to create 
autostereograms7.  Did they?  Here I report two examples of an apparent stereopair from the 
Book of Durrow5 which gives a sharp, strongly three-dimensional image that includes, among 
other symbols, an eye-shaped sign identified with mirages8 (Fig. 1).  Apparently, precocious 
empirical knowledge of stereopsis played more than a technical role in the creation of some of 
the world’s more famous graphic art.   
 Interlace and other regularly repeating geometrical patterns are characteristic of the Insular 
style of ornamental art, as seen in fine metalwork, jewelry, carved stone, and, following the 
introduction of Christianity and book production, illuminated  manuscripts4,5.  In the finest 
Insular gospel books, interlace suggesting flat braids and doilies of woven ribbon yields 
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stereoscopic images that resemble the actual articles in three-dimensional relief, but still qualify 
as wallpaper illusions6 (like the images formed by fusing the left and right X’s in Fig. 1d–f).  A 
few multiplexed designs, however, yield well-formed three-dimensional images based on visual 
disparities far too large to have been unintentional.   
 Fig. 1d–f shows a remarkable design that is repeated twice on the Book of Durrow’s folio 
125v.  The design can be seen as three large diagonal crosses (X’s), each outlined by four 
quarter-circles and centered on a small diagonal cross.  The design is laid out as an overlapped 
stereopair, with each half centered on a tiny, almond-shaped device, and with both halves sharing 
the middle X.  When the halves are fused, the stereoscopic image appears strongly three-
dimensional because the middle X is distinctly narrower than the other two.   
 Apart from the design’s layout, the most obvious indication that the devices in Fig. 1e, f are 
stereograms is that they were drawn accurately enough for their two halves to be easily free-
fused either by convergence or by divergence to form sharp, well-defined stereoscopic images.  
This is noteworthy because the devices contain curves that could not have been drafted with 
Insular artists’ standard compass, straightedge, and dividers alone (see ref. 9, pp. 214-226, 291-
296).  Perhaps the best examples are the non-circular arcs defining the central X’s.  With Fig. 1f, 
the disparities between corresponding points in some regions are large enough that one must 
change the vergence of the eyes slightly to take in the entire image by “focusing” up and down, 
as with Fig. 1a.   
 The stereoscopic image itself can appear in either of two ways, depending on how the 
viewer visually associates the arcs outlining the middle X.  If the arcs are seen simply as defining 
the X, the stereoscopic image appears as two X’s, joined at their tips and hinged along the line 
where their respective subparallel planes intersect.  If, on the other hand, the arcs are seen as 
Cisne_Stereogram Page 3 of 7 
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halves of four intersecting semicircular arches, less obvious shapes with intriguing symbolic 
associations emerge.  The “almonds” become the “pupil” of a three-dimensional “eye,” and the 
margins of the “eye” continue outward to form an outline approximating the sign in Fig. 1c, 
which is one of the very few symbols for mirage in Western ideography (ref. 8, pp. 73, 339, 
413).  Considering the uncertain semiotics of Insular art10, this and other features of the 
stereoscopic image allow considerable room for interpretation; however, the occurrence of a 
symbol for optical illusion actually within an optical illusion of a stylized “eye” seems hardly 
coincidental.   
 If the devices in Fig. 1e, f represent stereograms, who would have been the audience for 
stereoscopic art?  Presumably it would have included the illuminators themselves and likely 
other monks in the family of monasteries begun by St. Columba of Iona, a prodigious scribe 
known for miraculously rapid, well-witnessed, letter-perfect proofreading (ref. 11, pp. 284, 352-
353, 378) suspiciously similar to what editors can achieve with free-fusion stereocomparison6.  
The reason for associating the audience with this particular monastic tradition is that the 
illuminated manuscripts in which the microscopic details of stereoscopically fusible designs 
were drawn freehand are all associated with monasteries of this group, namely, Durrow, Kells, 
and Lindisfarne4–6.   
 What could have been the significance of stereoscopic art for early medieval monks, and 
what could have been the incentive for creating it?  Art historians have suggested repeatedly that 
Insular gospel books were focal points in the lives of their communities, and that full-page 
spreads of carpet-like ornament in Insular manuscripts (carpet pages) such as folio 125v were 
meant for prolonged and intensely focused contemplation12–14.  Pirotte14 noted the “wave-like 
motion” of the “animated surface” she experienced while fixing her gaze on carpet pages, and 
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elaborated on the significance Insular monks might have attached to similar experiences.  
Viewers can repeat the experiment with the stereopairs in Fig. 1, and perhaps notice similar 
sensations associated with instability of the eyes’ vergence and the incipient breakdown of a 
normal (non-stereoscopic) image (ref. 2, pp. 42, 46f, 388-390, 401).  Because the stereopairs’ 
offset is relatively small, ~11 mm, normal matching can give way easily to stereoscopic 
matching, even in the uninitiated (Fig. 1e, f).  Everyday phenomena no more remarkable than the 
resulting stereoscopic image passed for miracles among Insular gospel books’ original audience, 
as described in the Venerable Bede’s contemporary Historia Ecclesiastica Gentis Anglorum 
(731)15 (ref. 16, pp. xxiii-xxviii).  Here perhaps was the appeal.   
 A concatenation of suggestive and seemingly unlikely coincidences sustains the conclusion 
that Insular artists used an empirical understanding of stereoscopic images not only to create 
microscopically detailed ornamental patterns with the unaided eyes6, but also to explore the 
possibilities of the stereoscopic third dimension for ornamental art in ways not encountered again 
until the advent of crystallography (“three-dimensional ornament”)17,18 and the autostereogram7.   
 
 1. Wheatstone, C.  Contributions to the physiology of vision — Part the first.  On some 
remarkable and hitherto unobserved phenomena of binocular vision.  Phil. Trans. R. Soc. 
Lond. 128 371–394 (1838).   
 2. Howard I.P. & Rogers B.J.  Binocular Vision and Stereopsis (Oxford Univ. Press, 1995).   
 3. Wade, N.J.  A Natural History of Vision (Cambridge Univ. Press, 1998).   
 4. Alexander, J.J.G.  Insular Manuscripts from the 6th to the 9th Centuries (Harvey Miller, 
London, 1978).  
 
Cisne_Stereogram Page 5 of 7 
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 5. Meehan, B.  The Book of Durrow: A Medieval Masterpiece in Trinity College Dublin 
(Town House, Dublin, 1996).    
 6. Cisne, J.L. Stereoscopic comparison as the long-lost secret to microscopically detailed 
illumination like the Book of Kells’.  Perception 38, 1087-1103 (2009).   
 7. Tyler. C.W. & Clarke, M.B.  The autostereogram.  SPIE Proc. 1256, 182–197 (1990).   
 8. Liungman, C.G.  Symbols: Encyclopedia of Western Signs and Ideograms, 3rd ed (HME, 
Stockholm, 2004).    
 9. Brown, M.P.  The Lindisfarne Gospels: Society, Spirituality and the Scribe (British 
Library, London, 2003)  
 10. Thomas, C.  in From the Isles of the North  (ed Bourke, C) 1-7 (HMSO, Belfast, 1995). 
 11. Sharpe, R., trans.  Adomán of Iona, Life of St Columba (Penguin, 1995).   
 12. Ó Carragáin, É.  in The Book of Kells  (ed O’Mahoney, F.) 398-436 (Scolar Press, 
Aldershot, 1994).   
 13. Farr, C.  The Book of Kells: Its Function and Audience (British Library, London, 1997).   
 14. Pirotte, E.  in Pattern and Purpose in Insular Art (eds Rednap, M., Edwards, N., Youngs, 
S., Lane, A. & Knight, J.)  203–207 (Oxbow, Oxford, 2001).   
 15. Colgrave, B. & Mynors, R.A.B., eds.  Bede’s Eccesiastical History of the English People  
(Oxford Univ. Press, 1969). 
 
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 16. Wallace-Hadrill, J.M.  Bede’s Eccesiastical History of the English People: A Historical 
Commentary  (Oxford Univ. Press, 1988).   
 17. Weyl, H.  Symmetry (Princeton Univ. Press, 1980).   
 18. von Laue, M. & von Mises, R., eds.  Stereoscopic Drawings of Crystal Structures 
(Springer, Berlin, 1926).   
 
Supplementary Information is linked to the online version of the paper …. 
Acknowledgements  I thank Christopher Tyler and Nick Wade for their generous help and 
encouragement.  Any errors are mine.   
Author Information  Correspondence should be addressed to john.cisne@cornell.edu.   
 
  Page 7 of 7 
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Figure 1 | Explanatory sequence of stereopairs.  a, Stereopair based on one of Wheatstone’s 
originals (ref. 1, fig. 10): free-fusing the two subparallel lines with the unaided eyes reveals a 
strongly three-dimensional image of a line piercing the center of a circle.  b, Stereopair in which 
three devices like the pair in a are overlapped like those in d–f: free-fusing the two parallel lines 
reveals two circles that intersect at two points along the line of their respective subparallel 
planes’ intersection.  Owing to the overlap, the three-dimensional stereoscopic image continues 
seamlessly into the two-dimensional ghost images on either side, as in d–f.  c, An eye-shaped 
symbol for mirage8, a motif in d–f.  d, Schematic of the overlapped stereopairs in e and f: when 
the two tiny, almond-shaped features separating the three large diagonal crosses (X’s) are free-
fused, a strongly three-dimensional “eye” appears with the “almond” as its “pupil,” and the eye-
shaped symbol for mirage in c stands forth in outline, giving a symbol for optical illusion within 
an optical illusion of a stylized “eye.”  e and f, Tracings of panels from the lower (e) and upper 
(f) halves of folio 125v in the Book of Durrow (~680 CE)5, natural size, and in the original black 
and white: in each case, free-fusing the tiny “almonds” between the X’s reveals the symbol for 
mirage (c) within a stereoscopic illusion of an eye, as with d above, but now with two small 
diagonal crosses suspended within the image.  The small irregularities probably are due mainly 
to differential shrinkage of the parchment, which the regularly repetitive ornament covering the 
rest of the page confirms as relatively minor.    
10 mm
Figure 1 | Explanatory sequence of stereopairs. a, Stereopair based on one of Wheatstone’s
originals (ref. 1, fig. 10): free-fusing the two subparallel lines with the unaided eyes reveals
a strongly three-dimensional image of a line piercing the center of a circle. b, Stereopair
in which three devices like the pair in a are overlapped like those in d–f: free-fusing the
two parallel lines reveals two circles that intersect at two points along the line of their
respective subparallel planes’ intersection.  Owing to the overlap, the three-dimensional
stereoscopic image continues seamlessly into the two-dimensional ghost images on either
side, as in d–f. c, Eye-shaped symbol for mirage, a motif in d–f. d, Schematic of the
overlapped stereopairs in e and f: when the two tiny, almond-shaped features separating the
three large diagonal crosses (X’s) are free-fused, a strongly three-dimensional “eye” appears
with the almond as its “pupil,” and the eye-shaped symbol for mirage in c stands forth in
outline, giving a symbol for optical illusion within a stereoscopic illusion of a stylized
“eye.” e and f, Tracings of panels from the lower (e) and upper (f) halves of folio 125v in
the Book of Durrow (~680 CE), natural size: in each case, free-fusing the tiny almonds
between the X’s reveals the symbol for optical illusion (c) within a stereoscopic illusion
of an eye as with d above, but now with two small diagonal crosses suspended within the
image.  The small irregularities probably are due mainly to differential shrinkage of the
parchment, which the regularly repetitive ornament covering the rest of the page confirms
as relatively minor.
a
b
f
d
c
e


An apparent medieval stereogram incorporating a symbol for optical illusion

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An apparent medieval stereogram incorporating a symbol for optical illusion

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