more (at source): The monks could then refine any disparities by minimizing the apparent vertical depth of the images -- ultimately replicating the design element to submillimeter precision. Cisne proposed the idea in the July 17 issue of the journal Perception (Vol. 38, No. 7).

The paper suggests that the technique, called free-fusion stereocomparison, which takes advantage of the brain's ability to perceive depth by integrating the slightly different views from each eye, was known nearly a thousand years before it was articulated by stereoscope inventor Sir George Wheatstone in the 19th century.

Cisne analyzed the most detailed illuminated manuscripts of the Middle Ages, created between 670 and 800 A.D., including the Book of Kells (circa 800 A.D.); some have as many as 30 lines per centimeter.

Amazing!