Lincoln Cathedral, South Rose Window, Anonymous: The Bishop's Eye (1330s)

The Independent's Great Art series

A rose is a rose is a rose.

But for anyone whose idea of a two -dimensional image is a relatively small rectangle hanging on a wall, the rose windows of Gothic cathedrals present an unusual kind of picture.

They're enormous. They're seen from a great distance. They're seen at an angle, not face to face, but from below. And they're radiant, lit from the back, and often glowing out of near-darkness. Put these factors together and you have something that is, in fact, not so far from our experience. Nothing like a painting, it's quite like a cinema, or an enormous video screen.

Then differences begin again. These windows don't make the world huge as cinema does. Relative to the whole image, the pictures are miniature, and often hard to grasp. The idea of stained glass as the bible of the illiterate, a giant visual aid, must often have been a hopeful exaggeration. It would have taken a sharp and educated eye to decipher the images.

But what makes rose windows most alien to the modern viewer is the segmentation and complexity of their structure, their mixture of order and irregularity. They're made up, not of uniform pixels, but of units within units, levels within levels, from their containing circles down to their small, random-shaped, chunks of coloured glass, the whole pattern mapped out and held together by a framework of stone tracery and lead. As much as the images within them, it's the structure of these mighty diagrams that carries their meaning - and sometimes leaves us guessing.

Two great rose windows face one another across the north-south transept of Lincoln Cathedral. The north rose is known as the Dean's Eye and the south rose as the Bishop's Eye. They were first made in the early 13th century. They are described in the contemporary Life of St Hugh of Lincoln as "the two eyes of the church".

As "north represents the devil, and south the Holy Spirit," it goes on, "it is in these directions that the two eyes look. The bishop faces the south in order to invite in, and the dean the north in order to shun; the one takes care to be saved, the other takes care not to perish. With these Eyes the cathedral's face is on watch for the lights of Heaven and the darkness of Oblivion."

That's one reading, anyway. But this interpretation may not fit with what can be seen today. The Life of St Hugh was written in around 1225. About a century later the south rose window, the Bishop's Eye, had to be reconstructed. And while the Dean's Eye has a concentric layout that could be considered ocular, the Bishop's Eye is now not so eye-like.

The Bishop's Eye is unlike the great majority of Gothic rose windows. It doesn't have a revolving structure. Rose windows perform countless variations on the revolve. Their parts are by turns arranged like planets in orbit, spokes of a wheel, radiant beams, exfoliating petals, in cross formations, in star formations - but always the pattern is rotatable. It is focused on the centre point of the circle, and usually there is a central section at this point, the window's heart or hub. These windows picture an eternal, circling, radiating, God-centred universe. Not so the Bishop's Eye.

To put it geometrically: the circle of this window has been intersected by two arcs with the same diameter. The centres of these arcs lie on opposite sides of the window's circumference. The two arcs touch at the window's centre. They create a pair of upright almond forms. Each almond is then bisected vertically - almost all the way - by a straight line. Within these forms there's a network of tracery, its shapes mainly irregular long-tailed quatrefoils.

So the layout is essentially binary. Each almond is symmetrical around its vertical axis. The two almonds are symmetrical with each other. Indeed the whole rose (including the bits not within the almonds) is symmetrical around the vertical axis. So, although circular, this rose window does not hold a vision of centred order, nor a snugly interlocking yin-yang either. It presents a balanced division of two separate equal parts, left and right. The Old and the New Testaments, perhaps? The saved and the damned?

But students of symbolism will already be hot on another trail. Because this almond form, the vesica piscis (fish's bladder) as it's known, has an extremely versatile symbolic repertoire. The "measure of the fish" was considered a mysterious and sacred shape by Pythagoreans. Among pagans it could signify the vagina. Among early Christians, as the "Jesus fish", it signified Christianity itself. And in Christian art, as the Mandorla, it was an aureole or cloud of divine glory.

Meanwhile, students of the bleeding obvious can see in the Bishop's Eye a couple of leaves. The reason why the window is non-rotatable is that, fundamentally, its structure is not cosmic but biological. It spreads, in the manner of vegetables, in a single direction: upwards.

Within the almonds, the tracery ramifies like the veins of a leaf or the branches of a tree. The whole formation is cellular. You might even wonder if this binary window holds the secret of life itself - the basic reproductive dividing of cells.

Whichever reading takes your fancy, the imagery of the window won't help. It's lost. The stained glass was smashed out during the campaign against "abusive images" in the English Revolution. The pieces survived. But when they were returned to the window, in the late 18th century, they were put back higgledy-piggledy. The restorers didn't try to jigsaw the original images together. They had regard only to effects of colour and radiance.

This is common practice in English stained-glass restoration. If you can get up close to the window, you see a prophet's head spliced against a horse's tail, and an angel's wing and a shard of non-specific blue, and you try to make these fragments work as a consistent scene. But you can't do it.

Yet this mixed-up language is somehow familiar. Yes, it's collage. It's Cubism.

Out of one art of fragmentation, another is born.