Leaning on a rainbow, God promises Noah not to flood the world again. The Biblical bow at Chartres has stripes of red, yellow, and green, to imitate natural colours as far as size and technique permit. But Robert Grosseteste, the bishop of Lincoln, stated "it is manifest that it is not within the ability of painters to reproduce the rainbow". In truth, he was repeating a remark to similar effect, found in Aristotle's "Meteorology". Grosseteste argued rainbows varied with the weather, while painting produced a fixed result. The makers of the Noah window had no such qualms - coloured lights streaming through stained glass could ably mimic rainbow effects. The splendid colours of cathedral windows were admired by Saint Thomas Aquinas, who also rendered many Aristotelian ideas comfortable to Catholicism - including ideas as to the nature of light, the function of the senses, and the role of colour. But he bowed to Aristotle's dictums that the rainbow could not be painted, and that its proper colours were red, green, and purple. Most 13th century authorities on optics concurred: Witelo's "Perspectiva" (a source for Kepler's later studies) gave the same three colours, and John Pecham, the Archbishop of Canterbury, proclaimed no painter could reproduce the noble colours of the rainbow. Despite the pundits, it was a popular subject in murals, mosaics and manuscripts, from the early days of the church to the Renaissance.
Glass was made in ancient Egypt and Mesopotamia as early as 3000 BC, and the craft spread throughout the eastern Mediterranean. 'Glass lentils', that uncannily resemble crude lenses, have been unearthed at several sites - perhaps jewellers used them to magnify finely engraved work. In imperial Rome, the emperor Nero was rumoured to take a large emerald to sporting events; it has been suggested he used the gem as a monocle, to correct near-sightedness while watching the games. But eye-glasses, made with transparent lenses, are not truly known before medieval times. The ancient Greeks, however, understood how a glass lens could concentrate the rays of the sun, and the playwright Aristophanes described its power to melt wax. But the quality of glass was still poor a hundred years later, when Euclid wrote the first known texts on optics. He made no study of lenses, concentrating instead on a straight-line geometry of sight and reflection from mirrors, as well as magnification of objects underwater.
Improved techniques of manufacture under the Roman Empire produced clearer, cheaper glass. Spherical vessels were blown that, filled with water, could magnify print, and focus sunlight sufficiently to ignite cloth. In the second century AD, Claudius Ptolemy updated the rules in "Optics", measuring how the line of sight was bent, when refracted by glass as well as water. Others commented on the colours produced as a secondary effect of light's refraction - Seneca was charmed by rainbows issuing from glass rods, though Pliny the Elder thought their colours far inferior to those obtained with natural stones. Rock crystal, the most valued stone, could exceed man-made glass in clarity. In the 7th century BC, the Assyrian king Sennacherib had sat on a crystal throne in Nineveh. The Roman emperor Nero would later prize a tent made from rock crystal (and was devastated when the boat that carried it sunk). More practically, Greek medical men used crystal balls, to cauterise wounds with the concentrated heat of the sun's rays.
Fine crystals of quartz were preferred to crude glass lenses, for medieval eyeglasses. Others valued the stones for their ability to replicate the colours of the rainbow. In the 13th century, Roger Bacon obtained hexagonal crystals from Ireland and India, while Albertus Magnus got his samples from Germany and the region round the Red Sea. Their interest in rainbows was piqued by newly available scientific writings - Aristotle's "Meteorology" provided a starting-point, while Ibn al-Haytham's "Optics" (a weighty tome) was an authority on all things visual. Writing in the early 11th century, al-Haytham had replicated Ptolemy's refraction experiments for air, water, and glass. Medieval scholars began to trace the paths of refracted light rays in their passage from one medium to the next, until their empirical methods discovered a true cause of the rainbow. At the start of the 14th century, simultaneously in Saxony and Persia, Theodoric of Freiberg and Kamal al-din al-Farisi were directing light into spherical, glass flasks filled with water. Both men considered their flasks as individual raindrops, that bent the light that entered, reflected it from the back surface, and refracted it again on its passage back into air. By raising and lowering their 'raindrops', they were able to plot the angle at which a rainbow would be seen. (The secondary bow was traced to light reflected twice within the flask, before re-emerging.)
Though their observations were sound, the results were incomplete, especially regarding the cause of colours and general mathematical laws. It was only in the 1990s that 10th-century manuscripts were unearthed that contained these very laws. Working near the crystal mines of Basra, Ibn Sahl had hypothesised a sine law of refraction - much as we use today - for light passing through a lens. But history obscured the findings of Ibn Sahl, Theodoric and al-Farisi. René Descartes could re-invent rainbow theory in the 17th century, with the aid of a glass flask and his new-found sine law of refraction. Even so, precedence was quickly given to Willebrord Snel, and the sine law now bears his name. We now know Thomas Harriot had noted it down privately, even earlier in 1601, while James Gregorie would independently (though belatedly) make the same discovery. With carefully hand-ground prisms, Isaac Newton was to further analyse the bow and trace the origins of its colours. At the dawn of modern science in Europe, following the invention of microscopes and telescopes, the interest in optics was intense. Nevertheless, medieval men had pointed the way to their experimental methods, with observations on the play of colours; Roger Bacon was one to foreshadow the future direction of optics in 1263, with advice for a medieval crystal gazer:
"...let him hold these in a solar ray falling through the window, so that he may find all the colours of the rainbow, arranged as in it, in the shadow near the ray. And further, let the same experimenter turn to a somewhat dark place and apply the stone to one of his eyes which is almost closed, and he will see the colours of the rainbow clearly arranged just as in the bow."
Towards the end of the 18th century, Goethe refracted light through a prism, after masking different parts of the prism's face. (Witelo had tried the method, five centuries before, as had Descartes in the 17th century.) Goethe found colours emerged at the boundary where light met darkness, giving red and yellow at one edge, violet and blue on the other. As they spread out into space, the two edges merged into a greenish-white, or a 'peach blossom' colour, as the case may be. (Green is exaggerated in the drawing, to support Goethe's colour theory.) Holding the prism to his eyes, Goethe saw the same coloured fringes along edges to designs, painted on card. Roger Bacon had recommended similar observations in the 13th century, although Goethe abandoned the strictly mathematical approach his predecessor advocated. He even opposed the prevailing Newtonian view, that colours were components of white light separated by refraction. Instead, he postulated that white light, where it encountered darkness, was modified by 'turbidity' to produce colour. But Goethe lovingly observed colour effects others overlooked, and could justly criticize those optical scientists, who "over rather a prolonged period of time have tried to banish colour and rid their lenses of it".
In ancient Greece, Aristotle classified the theory of the rainbow as subservient to the science of optics, which in turn was a sub-branch of geometry. He considered the bow a reflection of the sun in a cloud, a kind of repercussion like an image from a mirror, or the echo of a sound. The water droplets in a cloud behaved like a shattered mirror, to reflect the sun's colour but not its form. The variety of colours arose as light and sight, already weakened by reflection and distance, met the darkening cloud. Aristotle limited the rainbow to three simple colours of red, green, and purple, and it is tempting to believe he chose them for optical reasons. After all, red, green, and blue-violet represent the primary mode of vision; added together as coloured lights, they can produce all the colours we see on the RGB screens of televisions and computers. But that division was first proposed by Thomas Young, early in the 19th century. Most likely, Aristotle sought a simple symmetry, in accord with the precepts of Plato, whereby any two opposites required an intermediary third to unite them. Aristotle's rainbow was reduced to its two outer colours and the one in the middle, since "three completes the series of colours (as we find three does in most other things)". Aristotle was obliged to explain away the obvious yellow or orange band. It was contrast, he said, that made red look 'whiter' next to darker green, and compared to the black cloud. He likened the effect to simultaneous contrast, whereby purple thread appeared differently on black or white cloth. Still, many later commentators were to adopted Aristotle's three-colour description unquestioningly. They were more interested in the cause of the rainbow, its shape and position, and in following the geometry of light and vision between the sun, the cloud, and the observer's eye. Colour was a secondary quality, a side effect of the interplay between light and darkness. Moreover, rainbow colours were ephemeral, unattached to any object, and shifted in the sky when the observer moved.
Around AD 200, Clement of Alexandria considered taking the likeness of the rainbow in colours to be as impossible as making a model of the sun, or capturing the appearance of God with an idol. The bow was an object of light, unlike the material surfaces of mere matter, let alone any facsimile in paint. Nevertheless, the rainbow became a popular subject in Christian art: most representations were fanciful, some were coloured naturalistically, but Aristotle's red, green and purple combination was not in evidence. It was theorists who followed the philosopher's scheme, made available in western Europe through 12th century Latin translations, from Arabic and the original Greek. Roger Bacon, for one, named red, green, and a blue as the irreducible colours of the rainbow. Like Aristotle, he gave numerological arguments in favour of the number three, adding it represented the Holy Trinity. Bacon next ordered them, equally spaced, between black and white, to make a set of five 'as nature intended'. He allowed that blue might be further subdivided into dark blue and purple, and green into different shades (though he avoided naming yellow, proscribed in Aristotle's bow). Building from three, to five, and finally to seven colours, Bacon reconciled the description in Aristotle's "Meteorology" with another list found in his "De Sensu", where seven colours (unrelated to the rainbow) were ranked in order from dark to light.
Rainbow lore was popularized in "On the Property of Things", a mid-thirteenth century encyclopaedia intended for the village friar. The manuscript was translated into many languages and, even as late as the 15th century, underwent more than seventeen printed editions. Like most medieval encyclopaedias, it included a chapter on the rainbow. The author, Bartholomaeus Anglicus, presented the traditional viewpoint on this and many subjects, juxtaposed to a more contemporary outlook. At first, he gave the rainbow truly bizarre colours - red on the outer rim, followed by brown, blue, and green. They symbolized the four elements, fire, air, water and earth, respectively, as if the rainbow were a silhouette of the world, and the elements wrapped around it. As his source, Bartholomaeus cited the Venerable Bede, author of an 8th century encyclopaedia. (Bede, in turn, borrowed much of his material from a previous work by Isidore of Seville, who had named red, white, purple, and black, as the colours of both the elements and the rainbow.) Bartholomaeus then gave his modern view, on Aristotle's authority, of a three-colour bow. He dropped the brown from his four-colour version, leaving red on the outside, blue in the middle, and green at the inner edge. Blue and green are switched from their natural positions, so neither of Bartholomaeus's alternatives - the Aristotelian or the elemental - followed the natural sequence of rainbow colours.
A teacher addresses his attentive pupils, hoping to explain a rainbow which stretches incongruously across the night sky. Gaily disporting in stripes of blue, yellow, green, gold, and red, it is a creature of the imagination. He might be quoting from the words of Bartholomaeus, to the effect that no painter can paint the colours of the rainbow. Befuddled students would get no joy from the accompanying text, where two different descriptions of rainbow colours are found. Neither list is true to nature, nor do their colours tally with the bow illustrated here.
Robert Grosseteste pioneered rainbow theory at Oxford, in the first half of the thirteenth century. Roger Bacon, John Pecham, and Bartholomaeus Anglicus, all followed in his footsteps there, while continental writings on optics, by Albertus Magnus, Themon Judaeus, and Witelo, show his influence. Grosseteste wrote that changes in atmospheric conditions, and in quantity and quality of light, altered "the shape of the light rays" to create the many colours in any single rainbow. Relying heavily on Aristotle, he recommended a logical and mathematical approach in the quest to understand nature. Roger Bacon insisted on the use of geometry, too, and placed study of the rainbow at the centre of the new experimental science in "Opus Maius", of the 1260s. But underlying the rational study of nature was a metaphysical view of light, which Grosseteste esteemed as the prima materia, the original substance from which the universe was made. Since the first recorded utterance of God was "Let there be light", it held great significance in Christian, as well as Jewish theology.
In the esoteric practice of Kabbala, a doctrine of light was central. Like many Christians, Kabbalists believed the divine light was hidden from all but the righteous, and that a rainbow would herald the coming of the Messiah. As written down by Moses de Leon in the "Sephir Zohar", around 1280, creation began with a dark flame in the depths of the heavenly sphere. At first it was obscure, "neither white nor black, neither red nor green, of no colour whatever". As it gained size and shape, radiant colours welled from its centre onto everything below. Likewise, Grosseteste held creation began with light, radiating outwards, to fix the heavenly spheres and the four elements, from which all earthly matter was composed. The stability and harmony of the whole was guaranteed by four characteristics - form, matter, composition and the composite. As a result, proportions between the numbers one to four (as used by Pythagoreans) were "the only ones that produce harmony in musical melodies, in bodily movements, and in rhythmic measures".
Grosseteste's cosmology of light lent visibility to the harmony of the spheres, and gave a physical dimension to divine illumination. In the world of matter, he thought light was all-pervasive, "for every natural body has in itself a celestial luminous nature and luminous fire". Grosseteste extended Aristotle's view of light (as a potentiality in bodies, akin to the element of fire), giving it a larger role. Light participated in seemingly unrelated physical phenomena, such as sound. When a bell or other object was struck, its parts oscillated, sending waves of compression and expansion through the surrounding air. Ordinary air had finer constituents - the elemental air and, finer still, light itself. All participated in the wave-like propagation of sound, and Grosseteste concluded that "the substance of sound is light incorporated in the most subtle air". Light coexisted with sound, and was fundamental to its physical behaviour. For possibly the first time in the West, a wave theory of light was implied by Robert Grosseteste. Such a view was common in China, as early as the 2nd century BC, where light, sound, and other qualities like the chhi, were held to radiate in waves. But 13th century followers of Robert Grosseteste wrote little more on the parallel. John Pecham provided one exception in "Perspectiva communis", a standard European textbook on optics until the end of the 16th century.
Pecham compared the conduct of light to circular ripples, formed by a stone dropped in a pond. Light's effects, he wrote, were explained by the geometry of its rays. Bundles of rays radiated in cones, from every point of a luminous body, but were all essentially part of the one light. Just so, ripples were distinct geometrical entities but still part of the water. The ripple analogy was an old one, used frequently since the time of the Stoics, to visualize waves of sound travelling through air. Rarely was the metaphor applied to light. (Theophrastus, Aristotle's successor, had imagined the moon's light caused its halo as a wavelike motion in foggy air, like a ripple on a pond. More generally, Plato thought both sight and hearing were affected by movements of fire and air, spread abroad in circles.) The analogy came into full force from 1678, when Christiaan Huygens' "Treatise on Light" asserted that light and sound both spread the same way, "by spherical surfaces and waves", of which the pebble-in-a-pond was a simple demonstration. By the beginning of the 19th century, Thomas Young was able to explain wave interference in water, light and sound, as two pebbles cast into the pond at once.
Robert Boyle allowed rays of sunlight to enter a darkened room, through a hole in the window blind. As they fell on either side of a prism, he plotting the paths of light and numbered colours that emerged after one, then two, refractions (above left). Boyle thought "the Triangular Prismatical Glass" gave the same 'emphatical' colours as the rainbow, and that they were probably no different to ordinary colours. Isaac Newton followed a similar procedure for analyzing light, shown in his delightful sketch (above right). Light is concentrated by a lens before entering the prism, to emerge as a spectrum of colours spread on a vertical screen. Newton simplified them to the same five colours as Boyle, conceiving each as a differently-coloured image of the sun - red at the bottom and violet at the top. Some red is allowed to pass through a pin-hole in the screen to show that, even when refracted again through a prism, it remains the same red and does not break down into a further range of colours.
Calling it his experimentum crucis, Newton let others know "on this I chose to lay the whole stress of my discourse". Often as not, he retained the five-colour description - red, yellow, green, blue and purple/violet - which accords with major bands seen in naturally-occurring rainbows. Theodoric used four of them (joining violet to blue) to analyse the bow in the early 14th century: Albertus Magnus had earlier settled on a less serviceable triad of red, yellow and green. But five seemed what nature intended, as Roger Bacon put it in the 13th century. (Bacon's five colours included black and white, and he expanded his final number to seven.) Newton, like Bacon, started with five simple colours, then added orange and indigo for a total of seven - red, orange, yellow, green, blue, indigo, and violet, or ROY G BIV for short.
Simple prisms were sold as novelties at English country fairs, in the middle of the 17th century. It is even rumoured Newton conducted his first optical experiments with one of these; most likely, he ground most prisms himself, to ensure their accuracy. Newton used them to unravel a remaining riddle of the rainbow - its colours - and published a convincing theory in "Opticks", of 1704. But bothersome experiments, with prisms and lenses in a darkened room, were necessary to confirm his findings, and even enthusiasts found the results uncertain. Other methods for producing spectra - staring at the sun through a feather, or pressing one's eyeball with a bodkin - were downright dangerous. Most people depended on fleeting appearances of rainbows to illustrate his ideas. Otherwise, the sequence of red, orange, yellow, green, blue, indigo, and violet (ROY G BIV) painted a word picture of colours, in a book illustrated solely in black and white. The picturesque explanation might have died a quick death, but for rapid developments in the crafts of glass manufacture in 17th century England, that lent relevance to Newton's discoveries. George Ravenscroft's patent for lead glass, and the formation of glass grinders' guilds, led to production of cut glass artefacts for the domestic market. By the first quarter of the 18th century, chandeliers and crystal goblets splashed the walls and boards of the homes of the gentry with rainbows. Newton's "Opticks" provided general explanations for the magical phenomena, even detailing colours thrown off at the bevelled edges of looking-glasses. ROY G BIV became a convenient tag for listing the prismatic hues.
Indigo was quickly spotted as an interloper, but when questioned about it, Newton pleaded he had simply listed colours noted down by 'a friend'. In the Latin edition of "Opticks", four blues were named between green and violet, in the order thalassinum, cyaneum, coeruleum and indicum. Optical refraction and diffraction can indeed produce very distinct blues - including a darkly sonorous royal blue (or indigo), and a light, bright greenish-blue (or cyan). While not obvious in the rainbow, both these blues can be observed in the refractions of cut glass and crystal, under normal conditions of sunlight or artificial light. Of course, the difference is noticed when two extremes of the blue range are juxtaposed, without intervening hues to soften the contrast. If you hold a prism to your eye, and examine coloured fringes at the borders between bright and dark objects (such as around the frame of a window), cyan is plainly revealed. The spectrum is split in half by the light coming through the window and the normal green disappears, as if separated into its components of yellow and blue - yellow, along with reds, moves to one side of the bright expanse, revealing cyan on the other side of the window, as a fringe to darker, redder blue. Roger Bacon recommended such simple observations in the 13th century, and again, Goethe noted the coloured fringes to shadowy edges, around 1800. Neither relied on a narrow, focused beam of light, as Newton did in his experimentum crucis. But, in other experiments, two markedly different blues could be distinguished. When diffracted, light wriggled "with a motion like that of an Eel" around sharp edges and fine obstacles, giving coloured fringes to their shadows. Newton acknowledged indigo and a paler blue, and included both in his final account of the spectrum.
The guild which donated this window is personified by a proud dyer, displaying his 'master-piece'- a length of cloth dyed to specified standards, that granted him registration as a master dyer. Dyeworks for red and blue were strictly segregated, and a blue master-piece was more often required than one in red, from the 14th century on. So the red cloth in the stained-glass panel is appropriately relegated to the background, behind the blue. For 12th-century Europe, most blue dye came from the woad plant, though some indigo was imported through Venice. Marco Polo later popularized the exotic dye, with tales of the splendid coloured cottons of India. After 1498, ships were able to round the Cape, to return with cargos of the valuable dye, and indigo became the blue of choice throughout western Europe.
Indigo was once known as a natural dye-stuff of inky blue, made of a dried extract from indigofera plants. It was used world-wide for thousands of years, and valued for its unique ability to permanently colour both animal and vegetable fibres. Students of the Old Testament conclude indigo gave the blue cloths and threads, decorating the tabernacle and Solomon's temple. According to early Christian parables - from the apocryphal Infancy Gospels, especially the Arabic version - Salem the dyer kept a vat of indigo in his workshop at Bethlehem. Whilst playing there, the Christ child threw all the cloths into the vat, much to Salem's consternation. Instead of blue, all the piece was miraculously dyed, in the individual colours each customer required. Indigo dye was a little too expensive for common cloth, though it had some use as a blue pigment for painting walls and watercolours. The Romans had little taste for clothing in barbaric blue; nevertheless, indigo was valued almost as highly as the royal purple of Tyre. (Though the purple was made from shellfish and the blue from a plant, the two colours share very similar chemical structures and excellent dyeing properties.) The colour was named after its exotic origins in faraway India, but Mediterranean peoples understood little of indigo's manufacture - as was clear from Pliny's mention of "indicum, a production of India, being a slime which adheres to the scum upon the reeds there".
In 17th-century Europe, cakes of indigo arrived aboard ships from the Orient and the New World. Woad, the European blue dye from a native plant, could not compete with the superior, low-priced import. Local economies collapsed in woad-producing areas of France and Germany, even where indigo was prohibited on pain of death. Englishmen were well aware of indigo's importance to their own economy, and Robert Boyle subsidized vain attempts by the Virginia Company to establish indigo plantations in the Americas. More generally, Boyle credited dyers with expertise in colours, and Newton made much the same observation. But their theory of three simple colours - red, yellow and blue - alongside black and white, more likely originated from colours mixed on a painter's palette, rather than from the vagaries of a dyer's vat. Newton would test dry indigo pigment, noting changes to its intense and consistent hue under differently-coloured lights. Other rich painting pigments - cinnabar, ultramarine, bise, and red lead - were subject to the same test. So were leeks, perhaps because Plato and Aristotle had listed leek green among the basic colours.
Along with the other pigments, indigo provided a de facto colour standard, a status it commonly enjoyed until the 19th century. Readers of Newton's "Opticks" would have clearly understood the descriptive term indigo - they were possibly wearing the colour themselves. In 1897, natural indigo was replaced by an artificial equivalent, and blue jeans are now dyed with this ersatz indigo (they say it is biodegradable). The name is now little used as a colour term, and 'indigo' mainly survives to describe the sixth colour Newton inserted in the rainbow. Other colour names, originally used to describe clothing, have passed into common use. Scarlet and purple were types of cloth worn throughout the middle ages; they were just as likely to be dyed any colour, other than those their names suggest. Even Sherwood graine described a weave of cloth, not a colour. Still, it is difficult to shake the image of Robin Hood and Will Scarlet cavorting through Sherwood forest, in green and red tights.
Newton thought colour sensations hit the brain in regular pulses, carried from the eye and along the optic nerve in tiny waves of aether. Assuming a similar process took place in the auditory nerve, he saw no reason that comparable mathematical rules did not apply for judgements of sight and sound - indeed, for all the senses. The same proportions of harmony and discord could govern perceptions of both music and colour. His 1704 "Opticks" presented the spectrum as a visual equivalent to sound, uniting colours of the rainbow by harmonic relationships, like notes in a musical octave. The five simple colours - red, yellow, green, blue and violet - were given a musical tone each, while orange and indigo occupied the two remaining semitones. Arranged in the Dorian mode, they make a white-note scale on a keyboard, starting at D. Orange spans from E to F, indigo from B to C: since the two colours are a fifth apart, they were considered particularly harmonious. If, perchance, Newton had chosen any other intermediate colour - namely lime or turquoise, either side of green - musical symmetry would have been destroyed. White-note scales on A or G would result, with semitones - and intermediate colours - separated by a fourth. This was felt to be a discordant interval at the time. A century and a half later, Hermann von Helmholtz gave the same green-yellow and blue-green as much validity as indigo, or golden yellow. He relied on spectra produced by diffraction gratings; they spread the colours more evenly than primitive prisms, to reduce the indigo region. In addition, Fraunhofer lines found crossing the spectrum of sunlight, divided colour regions naturally without the need for words. Helmholtz grew impatient of Newton's artificial measures, contrived by analogy to music.
When colour and music are linked, sight and hearing (that perceive them), and light and sound (on which they depend), may have correspondences, too. To sort out the difference, Francis Bacon made a lengthy list "Touching the Consent and Dissent Between Visibles and Audibles", in his "Sylva Sylvarum" of 1627. Bacon's influence on the new sciences was acknowledged by members of the Royal Society, to whom Newton submitted his New Theory about Light and Colors in 1672. Subsequent criticisms by a prominent Society official, Robert Hooke, provoked Newton to flesh out his thoughts on colour and light, and prompted him to disclose the musical analogy. It was well know that sound moved as waves in the air, and Newton conceded light made waves, too. They were provoked in aether, a substrata of matter, finer than light, able to infiltrate the pores of all bodies, whether transparent or opaque. Aether waves facilitated light's passage in and around objects, promoted the spread of heat, and conveyed sensation to the brain. Light itself was not aether, nor a wave, rather corpuscles of differing sizes, whose impacts incited the vibrations in aether. (Red particles, being the largest, caused the longest waves, while small violet ones made shorter vibrations.) Light and aether interacted, but they were distinct entities. Others, like Huygens, thought light was the very movement of aethereal matter. Hooke gave aether a role in conducting the primary forces of nature - force, gravity, magnetism, and light (which travelled in waves like water). As late as the 19th century, Clerk Maxwell still insisted on a luminiferous aether to carry light waves in an electromagnetic field. Even Albert Einstein was unwilling to exclude some kind of ether from general relativity, though it was superfluous to his photon theory of light in 1905, and had proved undetectable by experiment in the 1880s.
The 13th-century light of Robert Grosseteste had been more akin to an aether, the finest of matter penetrating all other bodies, and it even partook in the motion of sound waves. It was omnipresent, like an electromagnetic field, and described in mathematical terms, but its origins were metaphysical. Newton's rays of light globules were every bit as hypothetical, based on the latest corpuscular theories, but only by analogy were they connected to sound waves travelling through the air. He called upon the metaphor of ripples-in-a-pond (as John Pecham had done in his ancient optical textbook), to show his idea of light was not such a difficult concept. Compounded of all the differently sized colour corpuscles, white light was similar to an organ with all pipes played at once, not unlike the uneven surface of a turbulent stream. When the same light met a prism, its impact caused ripples in the aether (like dropping a stone in a pond), and they spread like sound waves to aid refraction. In the eye, variously sized light particles made differing impacts at the retina, to send a variety of aether waves along the optic nerve. The brain distingued colours from their several vibrations in that aether, just as the ear picked discrete musical notes in the vibrations of air.
"May not the harmony and discord of Colours arise from the proportions of the Vibrations propagated through the Fibres of the optic Nerves into the Brain, as the harmony and discords of Sounds arises from the proportions of the Vibrations of the Air? For some Colours are agreeable, as those of Gold and Indico, and others disagree."
The word orange did not appear in the English language until the 1540s, and only gained full usage after the exotic fruits were introduced into England, under the Stuart kings. As a recent word, orange was optional. Newton alternated it with gold or golden (translated as aureus in the Latin edition of "Opticks"), citrius (underscoring the fruity connotations of the colour term), and 'yellowish-red' (the present dictionary definition of orange). His uncertainty was due to the flux of language, and vagaries in interpreting the colour. Unlike the blues of indigo and ultramarine, no natural pigments served as standard oranges. Orpiment, the closest, was a deadly sulphide of arsenic often described as golden, with a long history in painting. Newton used it in some experiments, as dry powder mixed with other prepared pigments, but had considered it a yellow since schooldays. His prismatic orange relied on musical ratios to define it, as a semitone gap between red and yellow. To convey an impression in words, gold and citrus fruit were evoked; of all the colour nuances between red and yellow, the orange looks closest to a happy median.
Another famous fruit - an apple falling from a tree - reportedly inspired Newton's gravitational theory (though Euler later added, it must have fell on his head). Newton's interest in the new arts of gardening was evident from a few books found in his library, including the curiously entitled "Fruit Walls Improved". The Royal Society itself was chartered to improve the general study of nature, while doctors and apothecaries, amateurs and nobles, could examine plants and study horticultural techniques in Physics gardens. Fascination for exotic flora grew alongside England's maritime expansion, but permanent 'winter gardens' were needed to house specimens in cold northern climes. Citrus trees and other exotics were wintered in tubs, inside glazed rooms and closed galleries at the sides of houses. The facilities were called orangeries in France, and 'orangery' first appeared in an English dictionary of 1679. When William and Mary ascended the English throne in the Glorious Revolution of 1688, they imported an outstanding plant collection of 2000 different species, gathered from around the world. Dutch experts supervised the building of hothouses, where the best of their thousand orange trees sat in Delftware pots. In 1701, Sir Christopher Wren was asked to rebuild the orangery at Hampton Court palace. Meanwhile, Newton was thirteen miles away, in London, preparing the final draft of "Opticks" for the printers. Every morning, he would breakfast on bread and butter, washed down with a sweetened infusion made from orange peel, boiled in water. (It was better than tea, Newton thought, for dissolving the phlegm.) After a busy day, I like to imagine Newton sought out the affable Wren - "one of the great Geometers of our time" - to talk of orange and orangeries.
The English crown had been offered to William of Orange, as leader of the Protestant movement in the Dutch Republic. He was so named after a town in the south of France, where his family had originated. Coincidence or not, orange trees had once been shipped north from Orange, to the rest of Europe. The orange tree was suitably emblematic of the House of Orange, and William's countrymen and supporters were noticeable by their orange ribands. (Dutch sports teams still dress accordingly today.) The colours orange and indigo had political and economic significance in 17th century England, when Newton listed them in the spectrum. Both words gained some later importance in the English language from their inclusion in ROY G BIV. But neither is extensively used, except as a label for the particular hue, or to designate a plant. Both are impoverished in associative meanings, compared to other colour terms (blue, for instance, can be affixed to blood, devil, moon, stocking, and streak). It may be argued they lack an identity, even that they are perceptually weak; that orange, for example, has less character than purple and green, the other secondary colours mixed from paints. It takes on more of the character of its neighbours, as an ill-defined border between the strongly-marked bands of red and yellow in the bow.
Both colour names, orange and indigo, originated in nature, specifically the plant kingdom - orange as a fruit, and indigo as an extract. They evoke a world beyond the narrow confines of a prism in a darkened room, a sunlit world where real rainbows appear in the sky. Violet, at the most refrangible end of the spectrum, was also named for a plant. Newton at first called it purple, among the five original colours of his spectrum. He later reserved 'purple' for mixed colours, where the opposite ends of two different spectra intermingled. Since prismatic lights "manifestly transcend their Colour in purity", violet deserved to be named for the constant and deep colour of the flower, plucked from nature. But naturalists in the field found ROY G BIV inconvenient. Red, yellow, and blue pigments, mixed in known amounts, became a preferred standard by which colours of the world were catalogued. Despite Newton, printers also resorted to red, yellow, and blue inks, to mechanize colour reproduction. More recent technology has modified these colours, in accord with spectral science, while the natural rainbow has continued to resound in the imagination.