Building began on the church of San Vitale in 526, when Ostrogoths ruled both Italy and the western Roman empire, with Ravenna as their capital. The octagonal building was so renown that Charlemagne, almost three hundred years later, based his chapel at Aachen on the same plan. Around the altar in San Vitale, a richly-detailed expanse of mosaics seems almost riotously coloured, at first impression. Closer examination reveals the use of colour is strictly controlled. Each item is shown in its local colour, divided off by its outline from differently coloured articles that surround it. Some variation is allowed - a patch of earth, for example, may vary from dark reddish browns to dun colour, and to lighter yellow ochres and creams - but the colours are clearly related, providing no strong contrasts of hue. Even the peacock, an archetype of changeable colour to the antique eye, is so depicted. Its blue breast may fade into black shadow and contain white highlights, but it is clearly separated from the differently coloured wings and tail feathers. Almost all ancient mosaics separated colours into discrete areas, the main exceptions occurring around Christianity's most sacred images. Diverse colours were blended within the one enclosed border; like a magical threshold, it announced the divine presence of Christ and his symbols.

Illustration 1 :
RAINBOW MOSAIC BORDERS, San Vitale, Ravenna, c.540.

The main altar at San Vitale, on the eastern side, is set in a presbytery encrusted with mosaics. Entry from the main body of the church is gained through a ceremonial arch, where a bust of Christ gazes down from the soffit (right). The border of the mosaic's medallion is of colours graded subtly from the outside towards the centre - dark red-brown, red, pink, yellow, green, and blue. In the middle is a single row of reflective silver tesserae, or tiles.
A similar medallion (below) is placed at the other end of the presbytery, on the underside of an arch framing the niche on the eastern wall. It contains a signum Christi on a gold ground, set in a gradated border. This time, the border is split, top and bottom, and the order of colours reversed in the left half - so blue-green forms the rim, and red is placed along the inner edge. The same treatment is given to medallion borders on each side wall of the presbytery, though these enclose crosses and are held aloft by angels. Yet another coloured border runs around the niche itself, where a triumphant Christ in purple robe is shown, seated on a globe of the world. Its border, too, modulates from red to blue across its breadth; but it is cut into a dozen segments along its length, so the direction of colour-change is reversed in alternate sections. And in the vault of the presbytery, a haloed lamb is surrounded by a garland tied with rainbow bands.

Similar multicoloured patterning can be found in earlier churches, as rainbow borders to interior dome mosaics - for example, in a fifth century Santa Maria della Croce in Casaranello (surrounding a gold cross afloat on a starry sky); and again at Hagios Georgios in Thessaloniki, built at the end of the fourth century (enclosing garlands of fruit with a bust of Christ, unfortunately destroyed, at the centre). Equally splendid as any at San Vitale, they too presage the images of divinity. But there are secular as well as sacred uses of multiple colours: a floor mosaic in the Palace of Theodoric (also at Ravenna) had a meander pattern, of red fading to white then darkening to blue across its width. Similar colours were unearthed at Pergamon, as a square border in a Hellenistic floor mosaic. Though predating Christian examples by 500 years and more, the ancient artisan showed the same attitude to colour change. So careful was he, that white stones were tinted with coloured cement to achieve the subtlest shades. Starting from a bluish-green on the outside edge, green faded to the palest yellow, which then darkened through three shades of salmon pink to deeper reds.

These examples from antiquity suggest colour-order systems were held in high esteem. The colour gamut spanned from reds to blues and greens, between opposing warm and cool colours, though the polarity was not understood this way for many centuries. The English artist Charles Hayter was perhaps the first to publish a colour-mixing diagram divided thus, in his "Introduction to Perspective" of 1813 (left). The three capitalized primaries of red, yellow and blue cannot be fabricated from other colours, but each relies on its own unique pigment. Mixed together, two at a time, they create intermediate colours which form a sequence, not unlike the rainbow's colours. It is evident that mixing paints creates a necessary progression, that also makes sense to the eye and to the mind. Followed from red, clockwise through orange, yellow, and green to blue, it is similar to the colour order used for the border of the bust of Christ at Ravenna.

In spectral sequences of colour, yellow would automatically fall near the centre - perhaps not an ideal arrangement for mosaics. It could easily create a sour note with the different yellow of any gold tesserae surrounding the mosaic. Nor could gold be substituted for yellow in the middle of a multicoloured border; its greater reflectance would throw the adjacent colours into muddy obscurity (except if used sparingly as a highlight). White could make a better, brighter centre and many artists were to take this option. Even in the spectrum-like Ravenna border, red lightens towards white, through shades of pink rather than orange, and the yellows seem to have a greenish tinge. Colour choice was carefully controlled by makers of mosaics; selecting from a broad range of ready-made coloured glass, they decided the array of tiles before the mosaic was set in place. Great care was taken to ensure that colour change proceeded in regular stages.

At first, individual tiles were laid square in parallel rows (as at Pergamon), creating successive bands of colour. Later mosaicists might key them together by crenellating the border between bands. A more sophisticated method was to set the tesserae point to point, in a diamond pattern. This way, each row penetrated the next, and each colour came in greater contact with its neighbour. The effect was to blur the boundaries between adjacent colours and/or tones, creating smoother-looking gradations. Archaeologists have since labelled this technique of laying tesserae 'the rainbow style', though scholars still debate whether mosaicists intended to portray the natural bow. In any case, a resemblance to spectral sequences resulted from deliberate choices. The way to produce rainbow effects had become systematized by the 12th century, and was described - alongside how-to-paint methods for faces, figures, and robes - in Theophilus Presbyter's "On Divers Arts".

Illustration 2 : "THE BAND WHICH REPRESENTS THE APPEARANCE OF THE RAINBOW",
Chapter 16 from "On Divers Arts" by Theophilus Presbyter, early 12th century.

Theophilus's technique for painting a rainbow are shown, top right, approximating the different colour combinations he recommended. Generally, contrasting colours are placed either side of the central white strip. Each colour is tonally graded as well, from light in the middle to dark at the outer edge. In practice, red-green contrasts were often used (top, second from right): a Christ in Glory, in the Gospels of Henry the Lion (below, right) provides a textbook example. Painted a mere fifty years after "On Divers Arts" first appeared, the central figure has a rainbow of red-white-green around it. It is the only multicoloured border on the page. Oddly enough, there is a 'real' rainbow in the picture - the arc on which Christ sits. In other manuscripts, this could also be many-coloured, though often as not it might be plain or patterned. Here, it is tan, as is the inner border and an unusual back rest that suggests a secondary bow.
In Theophilus's method, a colour was shaded in discrete steps, from three to twelve in number. More and more pigment was added to the white at the centre, until a pure colour was reached. This was then mixed with a related hue - for example, yellow ochre with red - to darken the band by stages, as it approached the black edge. The same method could be used to paint shading on columns, tree trunks, towers and other round objects. Then identical colouring was used either side of the central white, in the manner of a grey scale (top, far right). A monochrome, in a single colour like green, served the same purpose, as would a simple blend (say, from ochre to red) on both sides. Theophilus recommended both monochrome and full-colour rainbows for borders. In the 13th century domes of St Mark's, Venice, mosaic portraits of the four apostles have monochrome borders (two in reds, the others in blues), while a medallion of the cross, and Christ's bust over the altar, are given rainbow surrounds.

"On Divers Arts" is the earliest accurate guide we have to many ancient processes in the arts. Theophilus outlined how to paint, to make glass, and to work metal, activities that fell within the purview of European communities of religious. Measurements were given to construct workshops and major equipment like kilns, instructions supplied for making specialist tools, and proportions stipulated for components of alloys and solders. The manufacture of half a dozen pigments was detailed, from raw ingredients to final colour, in recipes so clear and practical that we could follow much of them today. When it came to painting, Theophilus supplied formulae for showing figures and robes, that modified a single colour with two degrees each of light and shade. White and black, added to the base colour, usually supplied the shading, though related colours could be used - the folds of an ochre garment, for instance, might be darkened with red. The system of Theophilus relied on the polarity of light and dark, while the use of opposing colours was reserved for rainbow bands. (An interesting variation occurred in flesh shadows, where reds and roses were neutralized and darkened by mixing with green.) In any one painted area, changes of tone and hue proceeded in subtle stages, to avoid abrupt contrasts.

Well before Theophilus, a similar colour-order system had emerged, of carefully graded tonal values in tandem with hue variation. Historical representations of the rainbow often placed red along one edge, opposed to green or blue on the other edge. They were linked by merging through an intermediary colour, that could equally well be white or yellow. But that was not all: rainbow depictions also varied from bright in the centre to dark at the edges. Modern colour systems are careful to separate tone and hue, an invaluable distinction in analysis of the visual scene. But in nature, as well as art, the two are united in everything we see. Many antique borders seem systematic descriptions of tone-and-hue combinations, but they were not comprehensive. One limitation was that mixes of red and blue could not be expressed, since they occupied opposite sides of the border. (Purples could be found elsewhere, on the robes of Christ or other powerful figures.) In any case, if violet were to follow blue as it does in a rainbow, it could be lost in the dark margins. Individual variations occur, interrupting the pattern with idiosyncratic colour and tone, even partially reversing the spectral sequence (so light blue might follow darker green, for example). It also must be admitted that rainbow-like borders and bows were in a minority - perhaps one in five cases was so orderly. Still, the pursuit of systems of colour and tone is evident from Greek times, and confirmed by the chapter Theophilus devoted to its theory.

"On Divers Arts" served as a guide to the conventions of painting, simple enough for any artist to follow. Theophilus did not seek to lay down general laws, but to transfer a God-given legacy of art, handed down from pious predecessors. In loving descriptions of the craft, he included recipes for making pigments, with approximate quantities of raw materials. Painted gradations of tone and, to a lesser extent, colour, were described without the benefit of measurement. Their subtle changes were to be judged sensibly, by eye; Theophilus was content to stipulate 'a little' or 'much' of a named pigment. Further precision would be superfluous to painters' needs, apart from the extra trouble of measuring quantities. In contrast, dimensions were crucial in the manufacture of musical instruments, described in Chapters 81 to 87 of "On Divers Arts". Theophilus stipulated numerical proportions for bells and organ pipes, to govern pitches of their notes, and reflect the style of music they could play. Instructions on casting small bells gave a list of their masses that, despite a couple of errors, compared to Pythagorean ratios. Matched to the diatonic divisions of the octave, Theophilus's bells could well have played a musical scale. The same method would not apply to larger tuned bells, where width is the primary determinate of their pitch. Theophilus accurately described their manufacture - though the industry was in its infancy - but he wisely avoided pitch-size formulae for larger bells.

Readers of "On Divers Arts" were referred to existing tables of dimensions, that prescribed sizes of small bells and lengths of organ pipes, to determine the note each produced. Circulation of standard sizes suggests a rough kind of universal pitch was being attempted, in Europe of the Middle Ages. However, comparison of extant old instruments suggest any such attempt failed: they show disparities of up to five semitones in their pitches, a maximum difference that narrowed only towards the end of the 18th century. What mattered was that music had an intrinsic mathematical structure, built into instruments themselves with more or less success. Vocalists, composers, and tuners were familiar with its principles, as written down in late-9th century manuscripts of music and theory, and perpetuated by liturgical traditions of plainchant. Musicians, at least on paper, were ever bound by universal rules, that set notes at strict intervals which could be measured on a monochord. Colour was handled more sensually, since no standard existed to measure relationships between colours objectively. Even today, tuning devices and fixed frets help musicians find positions of notes mechanically, whereas painters rely more on experience and the eye, to establish scales of tone and colour. Finely calibrated grey scales and colour gamuts are chiefly used to support the modern technologies of print and screen.

Illustration 3 : ICON, early 7th century.

Kept in the monastery of St Catherine at Mount Sinai, this icon was painted in encaustic (pigment mixed in wax) by an anonymous Greek or Egyptian artist. The Biblical person most alike that portrayed is the Ancient of Days, who appeared in Daniel 7:9 with hair like pure wool. But then his garment should be white as snow - not the gold hatching that originally covered the icon's brown - and his throne would be a fiery flame, rather than the rainbow shown. More likely, the icon is a generic image of Jehovah, who had proclaimed in Isaiah 66:1 that "the heaven is my throne, and the earth is my footstool". In this interpretation, heaven has become parallel arcs of blue, yellow and red, sufficiently blended to suggest intermediate green and orange. Its resembles the natural rainbow, except the colours are in reverse order as red normally occurs on the upper edge. The entire figure (including earth, heaven's bow, and starry sky) is enclosed in an almond-shaped frame, called a mandorla.
The theme was revived in 19th century England, in a stained glass panel made for St Neot's church in Cornwall (below). This time, the correct colour order was observed - red at the top, blue and violet below. Seven curves of glass follow the sequence of ROY G BIV, the recipe Isaac Newton prescribed for rainbow colours. Unfortunately, they make for a clumsy bow, rather too wide for its curve.

There is no Biblical verse that allows of a rainbow being used as a seat, but there are hundreds of holy depictions of it in every medium imaginable. For more than a millennium, it was one standard way to represent Christ in Glory or Majesty; Eastern churches frequently used the device in icons, to celebrate the feast of All Saints or to depict the Ascension, described in Acts 1:11. And, throughout Christendom, Jesus-on-a-bow presided over many a scene of the Last Judgement. Christ (more so than God) was the usual occupant of the rainbow throne, occasionally accompanied or replaced by the holy lamb. From the 13th century, the Virgin would sometimes sit beside her son (and one rare drawing shows the Antichrist perched atop a red and green bow). Under special circumstances, one of the four evangelists might be shown on a rainbow; more often, they appeared in symbolic form - as angel, lion, ox, and eagle - around the mandorla. (Faint traces of them remain on the St Catherine icon.) The format is preserved in one very early example, at Hosios David in Thessaloniki. Its late 5th century mosaic shows a beardless Christ on a bow, all enclosed in a crystalline sphere surrounded by emblems of the evangelists. The rainbow there seems coloured dark blue on the upper edge, shading through greenish-yellow to white in the middle, then to red on the underside. Once more the colours are reversed, an unnatural sequence followed in about a third of such illustrations.

A similar colour order was followed in a mosaic rainbow in Monreale Cathedral in Sicily, some 700 years later, where it is part of the Genesis story of the Flood. While Noah sacrifices after landing the ark, God hangs the rainbow in the sky as a sign of His covenant (or agreement) to never flood the earth again. No one sits on this most famous rainbow; it appears in an earthly landscape rather than in the abstract context of rainbow thrones. Nor was it a frequent subject - there exist at least a dozen rainbow thrones for every instance of Noah's bow. Even fewer of them attempted to look real; one is in the porch of San Marco's in Venice, showing stripes of red, maybe some orange, then yellow and green, in the correct order. But blue and violet stripes are missing - they would have made the bow disproportionately wide anyway - and no attempt was made to blend the colours together, by laying the tiles on the diagonal or other clever tessellation.

The nature of Noah's bow was the subject to complex theological argument. Victorinus, an Austrian bishop of the 3rd century, had typified it as green and red in colour. Green symbolized the past judgement by the waters of the Flood, which drowned all living things except those in the ark. Red represented fire for the judgement to come, when the world would be destroyed and good Christian souls taken off to paradise. Gregory the Great gave the argument greater force, when he reiterated the colour analogy in the 6th century, and Rabanus Maurus, Archbishop of Mainz, adopted the scheme three hundred years later. The two destructions of the world, by flood and fire, were originally compared in 2 Peter 3:6-7, and Victorinus had been particularly concerned to relate them symbolically. He equated the rainbow in Genesis 9:13, the first book of the Old Testament, to a final apocalyptic bow in the last book of the New Testament. In the latter (Revelation 4:3), a radiant bow surrounded a throne, where sat a man representing Christ. He took on the appearance of jasper and sardine stone, known as green and red gems respectively, in antiquity. (Though we know jasper as a yellow, red, or brown opaque quartz, Pliny listed it as a transparent green, sky blue, or even rose or purple stone. Sard or sardius was a red cornelian, then as now.) The throne itself - presumably a piece of furniture rather than a bow - was given no colour: Victorinus transferred the green and red gem-colours of the seated man to the surrounding rainbow, even though the Bible text had it "in sight like unto an emerald". He applied the same colours retrospectively to the rainbow in Genesis, that appeared after Noah's ark had landed.

Such interchange of symbols persuaded early Christians their texts were compatible with older scriptures of monotheism. Importantly, covenants or testaments - words that are somewhat equivalent in the New Testament - had to display some kind of consistency; they were binding agreements between God and man. Since Noah's bow was a reminder of an old promise, it could be believed the rainbow in Revelation forecast an equivalent Christian promise. The new covenant in Christ had to equal, indeed surpass those given to the Jews, as Paul emphasized in his epistle to the Hebrews. The symbolic nature of Christ's covenant was laid out at the last supper, when Jesus offered wine as the new testament in His blood. So the colour red could represent wine and blood for the remission of sin, as much as hellfire at the world's end. But Victorinus adopted the latter red, so his rainbow became a dire warning more than a promise of redemption. Nowhere did he suggest either rainbow was a throne itself; the later proliferation of images containing rainbow thrones remains unexplained. Nor do the many depictions of rainbow borders surrounding Christ (enthroned or otherwise), necessarily symbolize covenants. As a means to glorify a sacred person, these spectral aureoles may be generally related to the figure in Revelation. Their Old Testament prototype is not to be found in the rainbow of the Flood. but rather in the visionary imagery of Ezekiel 1:28. The earlier prophet saw a figure that seemed of amber and fire, seated on a throne of sapphire, and surrounded by a radiance:

"As the appearance of the bow that is in the cloud in the day of rain,
so was the appearance of the brightness round about.
This was the appearance of the likeness of the glory of the Lord."

Illustration 4 : ST JOHN ON PATMOS,
detail of right wing of the St John Altarpiece, Hans Memling, 1474-9.

Memling's panel shows the vision John received while exiled on the island of Patmos. It was unprecedented in depicting the story of Revelation in a single picture. Jesus and the lamb are enthroned at the upper left, encircled by a rainbow, with the emblems of the four evangelists at the foot of the throne. The elders, seated round about, play their 'new song' on a variety of musical instruments, while a second rainbow encloses the whole of the heavenly scene. Outside it, the apocalypse proceeds on land, at sea, and in the sky. Against the horizon stands a mighty angel with a face as the sun, feet like pillars of fire, "and a rainbow was upon his head". He gave John the scroll of prophecy to eat, to ingest its words just as Ezekiel had once done.
Christ is draped in green and red, the colours of jasper and sardine stone, in accordance with the text. On the other hand, the rainbow has nothing of its reported emerald brilliance; green is merely implied where the golden bows fade into a neutral (even bluish) grey. Greens elsewhere in the picture are rather dull, too: Memling may have neutralized them, to contrast story elements in red and yellow with the blues of sea and sky. The rainbows nevertheless appear natural, due to their precise curves, their brilliance, and the smooth colour changes. In this they belong to a long tradition of multicoloured borders surrounding sacred images, including the 6th century examples at San Vitale (Illustration 1).

Doomsday predictions seem ever with us. "A miserable World is growing horribly Combustible", warned Cotton Mather, the first American elected to London's Royal Society in 1713. Today we sound a similar alarm over global warming, another catastrophe standing between us and paradise. Around AD 1000, many people expected the world to end, so the apocalypse became a very popular subject in art. The Saxon king Otto III commissioned the Bamberg Apocalypse to commemorate the occasion. Its manuscript contains the text of Revelation, richly decorated with miniatures on gold grounds. Rather than attempt a panorama of the whole story, as Memling would do, the limners illustrated separate episodes. The clothed figure of Christ appears in several, seated on a rainbow (rather than on a throne surrounded by the bow). His feet are sometimes placed on the earth, at other times on a second, smaller rainbow. The image is contained within the borders of a mandorla, often surrounded by the symbols of the four evangelists - a format borrowed from traditional Christs in Glory. Colours are kept simple; the two bows and the mandorla border might all be plain gold, or in separate colours shaded from light to dark, but no attempt is made to replicate the rainbow's varied hues.

Even though Armageddon did not arrived with the passing of the millennium, the end of the world continued to be a popular subject. (I fancy the populace was being warned not to let its hair down, that the danger remained.) More material was gathered from the Gospels of Matthew, Mark and Luke, adding scenes of the dead rising from the grave, to be cast into hell or ascend to paradise. By the 13th century, Christ could be found presiding over a scene of hope and despair, seated on a rainbow, with his robes thrown open to display the wounds from crucifixion. Known as Doom paintings, murals of the Last Judgement covered the chancel arch in many English parish churches. Christ on the bow was a frequent feature, though the confining mandorla was usually removed, and the plight of the damned was sometimes comically portrayed. Its curved shape distinguished the rainbow throne, as well as any second smaller bow placed under Christ's feet. They could be drawn in simple outline, or filled with coloured stripes and patterns of great variety. In more parochial examples, colour may have followed local tradition, limited by the number of pigments available.

Christ shown on a rainbow was common in illuminated manuscripts, in northern Europe especially, and scenes of the Last Judgement graced walls of courthouses, reminding everybody of the need for good behaviour. Romanesque churches in France preferred Christs in Glory, free from the theme of the Last Judgement. Even the rainbow was usually omitted, replaced by a cushioned seat, while the canopied throne (similar to the one in Memling's St John triptych) proved fitting for the narrow, vertical tracery of stained glass windows. But French murals evolved a great variety of mandorla borders, of elaborate shape, pattern, and colouring. Medallion borders to sacred images were equally varied, and it is apparent that no unbending rules applied to colour use in these contexts. But significant instances emerge, among hundreds of varied examples from different countries and eras, to show a like-minded exploration of pattern-making, tone and colour. Whether or not they were aware of historic or contemporary practice, artists in those cases followed the logic of colour use. They reconciled the warm and cool colours, lying towards either end of the spectrum, by simultaneously lightening their tones as the hues approached each other.

Illustration 5 :
CRUCIFIXION WITH THE VIRGIN AND ST JOHN,
Canon of the Mass Missal, Pinturicchio, 1493-4.

Pinturicchio may have had the covenant in Christ's blood in mind, when he painted the unique combination of a rainbow and a crucifixion scene. His delicate rainbow's colouring seems taken from nature, but its setting is unrealistic. It was commonly understood, from Aristotle's "Meteorology", that a dark cloud was necessary to reflect the sun's rays. Instead, the sky seems unusually clear and the sun appears over a hill on the left, while it should be behind the viewer.
Early in the next century, Pinturicchio returned to the subject of the rainbow in the Piccolomini Library, Siena. His fresco, "Aeneas Piccolomini Leaves for the Council of Basle", shows the bow standardized, as an arch of transparent yellow-brown, with a darker centre line. It resembles another by Cosimo Rosselli (&/or others), in "Passage of the Red Sea" of the 1480s; Pinturicchio would have seen it on the lower wall of the Sistine Chapel, when he worked there himself. (Recent cleaning, however, has revealed Rosselli's bow to be more translucent and colourful than it otherwise seemed - stripes of green in the middle, red outside and yellow on the inner edge.)

In manuscripts of the northern European Renaissance, from the mid-15th century, some artists began to take their model from nature. Their meticulous attention to the world of appearances gave rise to realistic depictions of rainbows. They adhered to its colour sequence, often blending their paints so transitions between the colours were imperceptible. The natural effect was enhanced by bright or intense colours, and by omitting firm outlines at the edges of the rainbow's arc. Attempts were also made to render the bow transparent, thus emphasizing the miraculous aspect of the motif - the weight of God made flesh supported by an insubstantial phantasm of colour. Sometimes, the rainbow appeared in a landscape setting, albeit as a backdrop to a religious scene. Pinturicchio used it so, within a miniature of the crucifixion (above); Matthias Grunewald was to frame the head of Mary by a bow in the country behind her, in his Stuppach Madonna of 1517-19. But landscapes with rainbows were extremely rare in both secular and religious art - even as late as the 17th century, those of Rubens, and in Jacob van Ruisdael's "The Jewish Cemetery", were exceptions.

In religious art, the rainbow throne was fading from prominence as a new iconography emerged. Images of the bow had begun to lose their colour, often reduced to a pale or golden band with minimal indication of its stripes. So it appeared in a scene of classical mythology by Dosso Dossi, catering to the interests of his noble clients. No longer would sacred figures front the viewer in the time-honoured Byzantine fashion, sitting on a rainbow, and confined by borders to their heavenly sphere. They began to disport themselves more freely about the sky, walking among the clouds, with not a rainbow in sight. The new arrangement could be applied to the Last Judgement, justified by the gospels of Matthew, Mark and Luke. On the last day, mankind would see "the Son of man coming on the clouds of heaven with power and great glory". This glory of the resurrected Christ was shown as a burst of golden light, if at all. It no longer manifested in perfected colour, "just as variously coloured glass derives its splendour from the sun's radiance", as Thomas Aquinas had put it (evidently, drawing his analogy from 13th century stained glass). As the Bible was read afresh during the Reformation and Counter-Reformation, its stories took on a new appearance.

The Protestant John Calvin reinterpreted Biblical rainbows, in a mid-16th century commentary on Ezekiel. Calvin sanctioned the Aristotelian view that the colours of the rainbow were produced by reflection of the sun's rays from a hollow cloud, saying "it is not for us to contend with philosophers respecting the rainbow". The bow of Noah was "the effect of natural causes", and he could not agree that it had put in its first appearance after the Deluge (an opinion that persisted among the pious and the learned well into the 18th century). It was a pledge of God's favour, as were the rainbows from the God-given visions of Ezekiel and John the Evangelist - though Calvin carefully pointed out each prophecy had a different meaning. He had no doubt the later prophets had seen colours as specified in the texts, and was at pains to question previous translations of colour terms - from Hebrew to Greek, and in the Latin Vulgate Bible provided by St Jerome. Of all the created universe, amidst the magnificence of the heaven of stars and planets, a rainbow provided "the image of deity more clearly expressed". Its colours, and those of the gemstones mentioned, were meant to dazzle and attract the eyes of the prophets, and assure them that heavenly secrets were being revealed to them. Unlike Victorinus in the 3rd century, Gregory in the 6th, and Maurus in the 9th, Calvin gave colours no individual symbolic meanings. They served to raise men's minds on high, their exquisite colouring intimating an even greater glory that lay beyond the reach of the senses. Similar thoughts had been expressed almost two thousand years before, in "Phaedo", where Plato compared earthly colours and gemstones to their fairer counterparts in the upper realm.

At the time Calvin wrote, artists were already moving on. But for the previous thousand years, a male figure (fresh-faced, regal, or mortally wounded), ensconced on a rainbow, had proved itself a most successful Christian logo. It superseded earlier attempts to personify Christ as the sun god, riding in a chariot with rays of light issuing from his head (a mosaic fragment shows him thus in the catacombs of Rome). More importantly for early Christians, the persuasive image of the rainbow god upstaged a rival piece of propaganda - that of Mithras slaying a bull, advertising another cult that promised life after death. That no Bible passage refers to Jesus sitting on a rainbow seems to have bothered commissioners of artworks not one jot. It also seems too powerful and direct an image to be derived from convoluted theology, and it is possible the idea had folkloric origins. The rainbow figures large in myths and legends worldwide, and it is usually seen as a bridge between heaven and earth, a weapon hung in the sky, a snake god, and so on. Occasionally, it became a seat; in the ancient Kalevala epic of the Finns, for instance, daughters of the stars perch on the bow. While the Romans certainly knew of the Finns, it is possible that similar tales were told by peoples closer to Constantinople, in the region where the rainbow throne is first found. At Hosios David in Thessaloniki, the motif still appears fresh and original, at the start of a tradition that culminated with the sophisticated products of the northern Renaissance (below). Displaying the rainbow gave artists a chance to explore ancient colour arrangements in ways that are still valid, despite the paucity in colour science of the time. The fact that a man once sat on the rainbow may still not be entirely forgotten; perhaps some cultural memories linger on, even in the lyrics of Tin Pan Alley. In fact, I feel a song coming on…