The Book of Kells and similarly illustrated manuscripts of seventh- and eighth-century England and Ireland are known for their entrancingly intricate artwork -- geometric designs so precise that in some places they contain lines less than half a millimeter apart and nearly perfectly reproduced in repeating patterns -- leading a later scholar to call them "works not of men, but of angels."
But behind the artwork's precision is a mystery: How did illustrators refine the details, which rival the precision of engravings on a modern dollar bill, centuries before microscope lenses were invented?
The answer, says Cornell University paleontologist John Cisne, may be in the eyes of the creators. The Celtic monks evidently trained their eyes to cross above the plane of the manuscript so they could visually superimpose side-by-side elements of a replicated pattern, and thereby create 3-D images that magnified differences between the patterns up to 30 times.
Read the whole thing, it's fascinating.