In 1704, Isaac Newton, Master of the Mint, President of the Royal Society and the most renown natural philosopher of Europe, condescended to reveal the secrets of light and colour to the world. What insights he had uncovered in forty years of experiment and analysis were laid forth in "Opticks: Or a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light." Through its three Books, each of several Parts, the reader is privileged to an inside view of Newton's laboratory technique, starting with simple experiments with a glass prism that anyone might conduct at home. It is still commonplace for school children the world over to duplicate his procedures - shining a thin beam of sunlight onto a transparent, triangular glass and noting the standard set of colours that emerge from the other side. Taking the process one step further, the same colours (focused with a lens) can be sent through a second prism: lo and behold! the colours recombine into a beam of the same white light from which they originated.

At its publication, "Opticks" was the most definitive study of light and colour yet produced, and was to remain so for the rest of the 18th century. It was Newton's most popular work, and subsequent editions, including French and Latin versions, placed his theories before a worldwide audience. Several concepts that originated there took root in the popular imagination, where they still remain. The prism's ability to produce a spectrum from a single beam of white light is perhaps the most durable idea to emerge from "Opticks": the rock group Pink Floyd used an image of the event, on the cover of their hugely successful album "Dark Side of the Moon" (below). On the front, a prism is shown splitting a beam of light into its component colours. They stream across the centrefold, interrupted by pulses in the green band to represent the rhythm of the music, or the beat of a heart. Finally, a second prism reunites the colours into the single beam of white light, from which they originated. While many fans may have realized the graphic (by Hipgnosis) referred to Newton's work, not all would have noted that six colours emerge from the prism, not the usual five or seven found in "Opticks". (Orange is included, but indigo omitted.) Still fewer were likely to care that this contradicted Newton's ideas on colour music.

After a sunbeam passes through a prism, it may cast its light on a wall opposite. To describe any image of the sun, so produced, Newton coined the term 'spectrum' (from a Greek word for 'apparition'). The word is now applied specifically to that oblong streak of light, which shows a band of colours of infinite variety. To talk of this array, spread from sunshine, individual colours need be named. We may agree to call a specific location yellow, say, or green, but there are no clear boundaries along the spectrum's length; colours merge imperceptibly into one other. While there are minute black cracks (called Fraunhofer lines) that cross the spectrum at fixed points, these were not noticed for almost a hundred years after "Opticks" was published. In any case, they do not separate colours conveniently, according to common names or colour groups. Newton had to be content to describe colour with words and labelled drawings. "Opticks" is provided with four fold-out sheets of illustrations, containing some fifty-six clear, black and white diagrams to amplify the text. That none of these is coloured was due to the technical limitations of 18th century printing; metal plates or woodblocks could not supply the subtle blend of colours required, and hand-tinting would have been prohibitively expensive.

So Newton needed standardized labels to separate colours, one from the other. In his earliest research, he often referred to the spectrum by its extremities only (the red and blue ends), but shortly rendered it consistently with a fuller description. Five colour terms - red, yellow, green, blue and violet - were employed, to designate successive bands of colour. In "Opticks", they first occur in Book I, Part I, Experiment III, and Newton uses the five-fold description at least a dozen times more throughout the treatise. It is handily applied, specifying which area is effected in what way by the experiment under discussion in the text. By Experiment VII of Part I, Newton was providing more detail, and added orange and indigo to his five colours, giving seven - red, orange, yellow, green, blue, indigo and violet. The seven-fold division, ROYGBIV for short, appears a dozen times or so throughout "Opticks", about the same frequency as the five-colour description. (There are also a few hybrid versions of the two, mostly of six colours, as well as important examples of fuller sets of colours.).

The production of colours in rainbows is explained in Proposition IX, Book I of "Opticks" . Again, Newton employed the seven hues of ROYGBIV, this time caused by light refracted through raindrops rather than a prism. They constitute the first consistent colour list for remembering the fleeting spectacle of the bow. Even René Descartes, in essays attached to "Discours de la method" of 1637, treated its colours cursorily, though he once listed up to six - orange, yellow, green and blue, lying between red and violet edges of the bow. (He did, however, clarify the size of rainbows, and was the first to publish the sine laws which govern the way light bends on passing into transparent substances.) Commentaries on bows, from Aristotle on, had usually specified a mere four, three, or even two colours: arguments were advanced excluding others, such as yellow, as somehow unreal, which did nothing for the advancement of optics. And medieval painters had rarely portrayed bows realistically; any number of variously coloured and patterned parallel curves seemed to do. Today, some spectral arrays can be quite as lax: winning cyclists at the World Championships are awarded 'rainbow' jerseys, that have horizontal bands (from the top down) of blue, red, black, yellow and green. These are nothing like the real thing. (Perhaps they're on rainbow-enhancing drugs…) They are, in fact, the Olympic colours in a 'rainbow' configuration.

It is important that some definition of rainbow colours exists, though a set of five (red, yellow, green, blue and violet) might seem adequate for any casual observer. But ever since "Opticks", the colour initials ROYGBIV have been used as a mnemonic for teaching children the colours of the rainbow. NASA Space Agency maintains a web page to that purpose, hosted by a Mr ROY G. BIV; in Indonesia, a popular chant, me-ji-ku-hi-bi-ni-u, shortens the Malay words for Newton's colours. Folk riddles name the same seven hues in Estonia, where they have entered into the encyclopaedia's definitions of rainbows. Likewise, the Oxford dictionary (at least my 1976 edition) gives ROYGBIV as the 'conventional' description of rainbow colours.

Since light behaves similarly, whether outdoors or inside, Newton could study rainbow-like behaviour in the controlled environment of his laboratory, applying the new experimental techniques of natural philosophy. The effect of adding or subtracting colours to or from the spectrum, the differential displacement of colours when viewed through a prism - these and other effects were described in Part I of the first Book of "Opticks". But, other than in the introductory Axioms, detailed calculations were avoided until the last two Propositions. Here, he described his famous reflecting telescope that had gained him entry to the Royal Society in 1672. He set out to show (erroneously, as it happened) how improvements to refracting telescopes were limited, because of colour distortion from the lens. The previous prism experiments in "Opticks" provided Newton with the means for his proof.

A quadrant was used to measure the angle of incoming sunlight, as well as that after it passed through a prism. Measurements were taken of the spectrum cast, so the angle it subtended at the prism's face could be calculated. From his data, Newton could give fixed figures for the different colours at either end and in the centre of the spectrum. At a given angle of incidence for the incoming light, whose sine was 50, he found the sines for angles of emerging colours, to range between 77 (red) and 78 (violet), at either end of the spectrum. According to the fifth Axiom, the sine law of refracted light was applied to colour, giving each a unique "degree of refrangibility". This indicated how much a coloured ray of light was bent away from the vertical, when passing from one medium to another (such as out of the glass of a prism into air). In modern parlance, each colour is refracted to a different degree - red the least and violet the most. The ratios of sines, of incidence to refraction, are set; for example, red's index of refraction remains at 50 : 77, no matter if the direction of sunlight changes, as long as the two mediums were glass and air.

Newton's sums gave the first quantitative proof of his revolutionary theory of colour, in support of his rather more qualitative experiments. His lectures at Cambridge, around 1670, had covered similar ground, with a great theoretical emphasis only hinted at in "Opticks". But there were limitations to both mathematical and experimental approaches. Newton never settled on a single method to account for all the coloured rays; he also treated light as if it were only refracted once, not twice, on passing through two faces of a prism. Acknowledging the weakness of one such generalization, he wrote:

"Moreover, if all the other kinds of rays were considered simultaneously, that assertion, though no longer absolutely true, would still so closely approach the truth that it could be taken as true with respect to sense and a mathematical calculation. Consequently, since a geometric calculation of the refraction... can be undertaken rather difficultly, I will not be afraid to do it in a way which, however mechanical, is more suited to practice, being confident that no fault aught to be attributed to me if, when I perform computation in physical matters, I omit the minutiae, that entail burdensome and fruitless work."

Newton's quantitative approach to colour was carried over into Part II, Book 1, of "Opticks" where, for the first time, he established dimensions for each of the seven colours. In Experiment 7 of Proposition 3, Problem 1, he re-employed the figures from the section on telescopes, in Part 1. The sines for the extreme red and violet of a rectangular spectrum were again 77 and 78, for a sine of incidence of 50. All he had to do was fill in the intermediate colour divisions. Repeatedly, he drew vertical lines across the spectrum, where he felt the main colours were divided, checking them under different circumstances. In an attempt at objectivity, Newton asked "an Assistant, whose Eyes for distinguishing Colours were more critical than mine" to draw his own colour partitions. From the agreed dividers, Newton could well have calculated each sine of refraction but, in an extraordinary leap of imagination, decided to compare them instead to the notes of a musical scale:

"...in proportion to one another, as the Numbers, 1, 8/9, 5/6, 3/4, 2/3, 3/5, 9/16, 1/2, and so to represent the Chords of the Key, and of a Tone, a third Minor, a fourth, a fifth, a sixth Major, a seventh, and an eighth above that Key..."

Illustration 1 : MUSICAL DIVISIONS OF THE SPECTRUM, ”Opticks”, 1704. Book I, Part II, Fig.4.

The colour-music layout, shown above, delineated notes as vertical lines, with the key note on the left at AG. The chord, or string length, that gave every note its pitch, ran from G to M at the other end of the spectrum (covering the whole octave, or eighth), and was extended the same distance to X, providing a second, reference octave from M to X. AG gave the lowest note, with GX as the chord of greatest length; at the other end, the note FM determined a string half the length, MX, sounding a full octave higher. (By musical convention, the interval between the two is given by the ratio of their string lengths, GX : MX, or 1 : 1/2 = 2 : 1, the ratio of an octave.) Other notes are found by the fractional distances marked along the spectrum: the third note from the left, for instance, was given as a third minor, and lies at 5/6 of a string-length from X. (Its measured distance from the key note AG is decided by the ratio 1 : 5/6 = 6 : 5, which is the accepted standard for the minor third, in a musical system known as just intonation.)

The seven colours red, orange, yellow, green, blue, indigo and violet (ROYGBIV), fill the seven intervals between the eight notes, starting from the highest note on the right. In this instance, they form a scale running downwards from the high red, the least refracted light, to the deep violet, which is refracted the most. The colours are more cramped at the red, right-hand end, as are the musical notes. If one considers that the ratios for musical notes are really the same size at either end - both are major tones, of 9 : 8 in just intonation, as is the central green - the relative compression of red and expansion of violet are evident. The compression of higher intervals is a characteristic of musical scales; it occurs because their various ratios are successively multiplied, rather than added, together. (From the half string-length at extreme red, the next lowest note is obtained thus: 1/2 x 9/8 = 9/16, while 9/16 x 16/15 = 3/5 gives the following note, etc. To calculate notes from the low, violet end, the string-length need be divided by ratios, i.e., invert them and multiply: 1 x 8/9 = 8/9, and 8/9 x 15/16 = 5/6 give the two next ascending notes.) The compression is obvious by the spacing of frets on a guitar's neck, which grow closer together with ascending notes. The somewhat similar appearance of the spectrum may have influenced Newton, to suggest its likeness to music.

But in other experiments, with combinations of lenses or prisms, Newton got the opposite results. Tilting the lenses made the spectrum shrink into white: further tilting caused the colours to re-emerge, but in reverse order. (The spectrum could be likened to a fan, with each blade a different colour, that becomes white when closed, and reveals the reversed colours by opening in the opposite direction.) As early as 1665, Newton had recorded in his student notebook, "Of Colours", how "Prismaticall colours appeare in the eye in a contrary order", when using a tank of water to bend the light. In "Opticks", Experiment 7 with the prism was the only instance, using the musical analogy, where violet was more expanded than red, and so occupied the lowest musical interval. In the other six occurrences of colour music - with a colour-mixing disc, another mixing diagram, experiments with thin and thick plates, and the calculations of 'Fits' for both - red occupied the low end of the scale.

When red became more expanded than the violet, the intermediate colours swapped sizes too - orange with indigo, yellow with blue. The behaviour of coloured light still suggested a non-linear structure, similar to the known, geometric order of musical sound. But the whole colour music array - established in the diagram of the prism above - needed to be reversible, to accommodate colours in the opposite order, with red at greatest size. This Newton achieved with a symmetrical musical framework, selecting ratios of notes accordingly. They remained stationary, while the colours could then run either up or down the scale, as the case required.

Why did Newton decide to use musical measurements? The most succinct explanation is they filled a mathematical gap. As soon as he had established the arrangement in Experiment 7, Newton paralleled the fractions of the string lengths to sine values, thus eliminating the need to make tedious and repeated calculations for the sine values of every colour margin. Assigning 77 to red and 78 to violet, he divided their difference with the same fractions by which the spectrum was divided. ( The red -orange border, for instance, occurs at 9/16, which lies 1/16 from red at 1/2. This amounts to 1/8 of the spectral length, and of the 1/2-string length. Its sine value is therefore 77 1/8: successive fractions were added to gain sine values for all coloured borders.) Similarly, Newton manipulated the musical fractions in later experiments - to compare the colours proportionately, to scale the spectrum to a different size, to establish relative positions of colours, and to otherwise insert them in more complex calculations. It is noteworthy that he nowhere mentioned the musical ratios themselves, preferring to deal with abstract numbers for string-lengths, as if they were experimental measurements. So no idealized musical scale was posited directly - perhaps in the hope of avoiding the ire of speculative theorists in the field.

Whether the colour-music equation works for all Newton's applications would depend on the accuracy required. It certainly simplified the text, and provided a memorable emphasis to some important experiments and ideas. However, it implies a general theory about the relation of colour to music and, as such, it is fundamentally flawed. To equate a geometrical progression (musical notes) to a sine progression (values for refraction) is not possible, as they are not the same thing, and inconsistencies arise inevitably. The case established in Experiment 7 only applied in that one instance; as the angle of incident light changed, the visible spectrum would alter with it. At greater angles the spectrum lengthens, though at a decreasing rate, while the centre creeps away from the blue-green border towards yellow. The changes occur in the opposite way when the angle decreases. Moreover, the outer limits of the sines, red to violet, expand increasingly with greater angles and decrease more and more slowly as angles lessen. Their difference of 1 between overall sine values, insisted on by Newton, in fact changes by 10% if the light shifts by 7 or 8 degrees - a figure within Newton's experimental range.

It is worth debunking the colour-music association somewhat, considering the portentous finality with which Newton pronounced it, and the way many since have treated it as some kind of occult law. Much of it is knowingly untrue: the one verity, if each coloured ray behaves consistently, is that the internal sines retain their musical pattern, despite changes in size to the colours they represent. And even that does not depend on his musical divisions, since any arbitrary set-up would have behaved just as well. Newton partially conceded this, in his Cambridge lecture notes on optics, by admitting that an equally-tempered scale would not produce a noticeable difference, and that spectral colours merge anyway:

"Hypothesis explaining the Properties of Light", published in the Philosophical Transactions of the Royal Society in 1675-6, contained Newton's first attempt at colour music. It differed little from the final version in "Opticks", but the text was more tentative, remarking how the spectrum "was divided in about the same proportion that a string is, between the end and the middle, to sound the tones in the eighth." Instead of a formal list of string-lengths, and the sequence of notes it represented, an arithmetic construction was supplied: the spectrum was divided into abstract fractions of its length, though the same musical ratios would result. (An identical method was belatedly inserted into Newton's original lecture notes from Cambridge; when they were finally published as "Lectiones opticae", colour music would appear at Lecture 11.) The naming of notes was left to the diagram, whose sol-fa syllables would have sufficed for the learned gentlemen of the Royal Society. The terse description - along with the instructions for plotting the colours of thin plates - was later modified, for the more general readership of "Opticks". Neither does Newton link colour margins to their sines of refraction. Rather, the topic is mentioned only in passing, as the 'sine law', first put into print by Descartes in appendices to "Discours de la Methode" of 1637. Values of sines are only given for light generally, not single colours, in the earlier "Hypothesis"; they appear as ratios, indicating how light is differently "turned awry" by water, glass and air.

However, Newton prefaced the "Hypothesis" with a lengthy exegesis on his aether theories, replete with examples of the analogy between light and sound. The luminiferous aether was co-existent with the air, but of much subtler stuff, its particles being even finer than those of light. Aether vibrated with the passage of light globuli, and even did so within the eye to cause the sensation of colour. Likewise, air vibrations set off by a vibrating body - and strings, bells, organ pipes, trumpets and trunks were mentioned - caused the sensation of sound. The 'bigness' of the vibration, of either air or aether, dictated the pitch of a musical note, or the colour perceived. Rhapsodizing on the all-pervasive aether, Newton speculated on its various kinds, even a type that may be the original matter of Creation. Its role within the body might be governed by the soul, that willed the aether to expand or contract according to its 'spring' (or elasticity), so causing the muscles to move.

A week after this preface to the "Hypothesis" was read to the Royal Society, we find Newton thanking Robert Boyle for smiling at his proposal, although the latter had also accused him of "trepaning the common Aether". As it eventuated, the aether was to take a much more formal and confined role in "Opticks". Wilder speculations (on the part aether played in heat, fire, chemical reactions, and the composition of planets) were related to a broader theory of matter, and to the mutual attraction of bodies evident in magnetism, electricity, and gravitation. But these speculations were restricted to the Queries at the end of "Opticks”. The list of Queries was augmented in the Latin edition of 1706, and considerably expanded for the second English edition of Opticks, in 1718.

Illustration 3 : ORIGINAL DIVISIONS OF THE SPECTRUM,
"Hypothesis explaining the Properties of Light", 1675.

For the first time, Newton illustrated his concept of colour music with a diagram inserted in the "Hypothesis" of 1675. The drawing was more spontaneous and lacked the geometrical rigidity of the later one in "Opticks" (Illustration 1, above). The proportions were less accurate (indigo is considerably too wide) and not as idealized (the height of the later version was adjusted, so an end-circle, at the right, would artificially define the red-orange margin by its tangent). Absent, too, are the Greek letters and fractional dimensions, which were to give the illustration in "Opticks" a semblance of authority. But otherwise the two were the same, conveying identical information to express the selfsame concept. In many ways, the original in the "Hypothesis" is a more direct and successful graphic, thanks to its simplicity. The colours are labelled, and exact locations of their boundaries are given in the text by fractional divisions of the spectrum xy. (On the “Opticks” diagram, proportions of the total string length GX are more tellingly described as musical ratios.) Here, musical notes are indicated by sol-fa syllables, according to an idiosyncratic method common in England at the time. By naming both notes and colours, Newton made colour music explicit on this earlier diagram. Less so in “Opticks”, Figure 4, which left relationships between music, colour, and sines of refraction to be spelt out mathematically in the text.

The sol-fa system, or solmization, originated in the 11th century with Guido d'Arezzo; he expanded the basic musical measure (the tetrachord of ancient Greece) from four notes to six. Guido used six Latin syllables - ut, re, mi, fa, sol, and la - to name the ascending notes of his hexachord. Pythagorean whole tones, with ratios of 9 : 8, separated the notes, except for the middle interval of mi-fa, which was given the Pythagorean semitone of 256 : 243. These six notes formed a natural hexachord, starting on C and ascending to A. Two other types of hexachords were used - a hard one beginning on G, and a soft variety, on F; they included different B notes - natural or flat, respectively. Seven hexachords at different pitches were overlapped to form the ‘Gam’ scale, starting on the lowest note G, or gamma. It spanned two octaves and a major sixth, and a melody could meander through the Gam, 'mutating' from one hexachord to another. Singers might plot the path of a song on their hand, where note names were allotted to various finger joints. The Guidonian Hand became the standard way to pass on a tune, at first orally, then in writing.

Illustration 4 : THE HEXACHORDS
from “Compendium Musicae”, René Descartes, 1650.

By the Renaissance, changes had crept in to Western music. Ancient Greek texts, recently translated, showed the classical legacy to be more complex than previously imagined. A subtler understanding of music theory arose (due in part to Arab influence), while new instruments like the lute (al-'ud), had different tuning requirements. In "Musica Practica" of 1482, Ramos de Pareja called Guido “a better monk than a musician”, demanding a new system of solmization that covered all notes of an octave and allowed more accidental semitones. He also questioned the internal structure of the hexachord, the distances between individual notes themselves. Ramos would stipulate non-Pythagorean note ratios, changing the semitone to 16 : 15 and adding a minor tone of 10 : 9. Traditionalists were outraged, but the new ratios produced ideal musical tones for the voice and fretless instruments. With these, a chord of keynote, third and fifth could be sung in sweet harmony - a common enough practice, in fact, since John Dunstable in the early 15th century.

Gioseffo Zarlino incorporated the new ratios in "Institutioni Harmoniche" of 1558, finally overturning the ancient Pythagorean system. He extended the narrow framework of numbers 1, 2, 3 and 4 (and their multiples, by which the Pythagorean ratios were formed), to include the numbers five and six, in his "numero Senario". Citing Claudius Ptolemy's "Harmonics" of the 2nd century as his authority, Zarlino also restricted the form of note ratios to superparticulars. in these, the numerator of a fraction is one greater than the denominator. (His semitone of 16 : 15 fulfilled the requirement, the Pythagorean semitone of 256 : 243 did not.) The new semitone, along with major and minor tones, formed the basis of a system later known as just intonation. Confronted by this overhaul of music, the most comprehensive in a thousand years, theorists like René Descartes were kept busy trying to fit just ratios to the old Guidonian Hand - a task that Zarlino did not complete.

Just intonation was not perfect - it failed to produce all the ideal fifths and thirds it aimed for - but it suited the needs of the 17th century better than the old Pythagorean ways. Nor was it the only system around: its mathematical niceties were impractical for keyboard players, who preferred the approximations of meantone tuning. By tweaking the fifths a little, fluent transpositions could be achieved between most of the keys. And the very idea of keys was novel. It was presaged in "Dodecachordon" of 1547, where Heinrich Glarean posited two new modes to supplement the four ecclesiastical modes (Dorian, Phrygian, Lydian and Mixolydian). His Aeolian and Ionian modes are equivalent to the white notes of a keyboard and the modern keys of A natural minor and C major, with no sharps or flats in their key signatures. They rose to prominence in the 17th century; Newton surely noticed them when reviewing Francis North's "Philosophical Essay of Musick" in 1677. The scales of C and A divided all others into sharp and flat keys, and determined their sol-fa names. As musicians adapted to keys and scales, and the accidental black notes they entailed, theory began to bow to the inevitable. A certain democracy had arisen amongst the musical notes, and the revered Church modes were becoming obsolete.

Solmization would survive. Eventually, the first note was altered from ut to do or doh, for easier singing; an extra syllable ti (or si) was later added to form a seven-note scale. Latin names for notes are now firmly attached to the C major scale in many countries, where sol-fa syllables are interchangeable with the alphabetic labels of C, D, E, and so on. Otherwise, sol-fa can apply to any scale, as taught by Sarah Glover of Norwich in 1835. She instructed hymn singers in an updated sol-fa technique, replacing written music and alphabetic note names. Glover was delighted to discover Newton's allocation of colours to notes, "an analogy affording such beautiful evidence of design in the works of the Creator, and so clearly evincing the Divine Origin of Music". John Curwen soon adapted Glover's work as a successful teaching method; by the beginning of the 20th century, millions of children throughout the British Empire were instructed to sing by his Tonic Sol-Fa. Solmization has a present incarnation at the Kodály Academy, and the Guidonian system is further immortalized (for the time being) by Julie Andrews singing, “Doh, a deer”, in the Rogers and Hammerstein film, “The Sound of Music”.

Illustration 5 : THE GUIDONIAN HAND from Zarlino's "Institutioni Harmoniche" of 1558.

Zarlino included this chart for historical interest. It shows the medieval music system of Guido d'Arezzo, with overlapping hexachords of six notes each, arranged in columns. (The crescents on either side indicate the ancient Greek tetrachords of the Greater Perfect System.) Each note is given a sol-fa name, and connected to a list of letters on the left, which eventually became the notes' modern names. A simplified version of the chart appeared in "A Plaine and Easie Introduction to Practicall Musicke" of 1597, a popular English text by Thomas Morley. Colours have been added here, to approximate how Newton used the arrangement for his colour music.
For the bulk of "Opticks", the spectrum ran up the scale from the red D in the middle, jumping between hexachords, to the topmost violet. In the "Hypothesis" experiment with a prism, he started on the lowest violet, but ran up the scale to the same red D. The reversal of colours was implied by the prism experiment in "Opticks", where Newton omitted the sol-fa labels - presumably in a vain attempt to avoid contradicting the rest of the book. It is interesting that the lowest note named in Guido's system was ut at gamma (or G) and the whole range of notes was called the gamut (gamma-ut); the same term is applied now to colour arrays, in the computer and printing industries.

In 1675, Newton attached sol-fa syllables to colours of the spectrum in his "Hypothesis" (Illustration 3), using musical terminology and proportions that found their way into "Opticks" of 1704. He sought a symmetrical array of notes, to accommodate his experimental observations. According to the principles of the Guidonian Hand, a diatonic scale starting on D was the obvious choice. To find its sol-fa, Newton started from the note D in the hard hexachord of G. There, he obtained the first two notes, sol and la. Mutating to the natural hexachord of C gave him the following fa, sol, and la; the final mi, fa, and sol came by jumping to the next G hexachord. (The only alternative, mutating between the C hexachord, and the soft hexachord of F, would have given a symmetrical G scale that included a B flat: such a choice was contra-indicated by later remarks in "Opticks".) Newton’s scale was conservative, even for the time, and related to the first of the Gregorian Church modes, the Dorian. (In modern terms, lacking accidentals, it is equivalent to a C major scale but with D as its key note.) In his sol-fa, however, Newton followed contemporary musical conventions by using only four syllables – mi, fa, sol and la. The system, which omitted ut and re, was brought to England from Geneva in 1596. It became common practice, as Penelope Gouk highlighted in "Music, Science and Natural Magic in 17th Century England", quoting a popular ditty:

"The first three Notes above your Mi
Are fa, sol, la, here you may see;
The next three under Mi that fall,
Them la, sol, fa you aught to call.
If you sing true without all blame,
You call all Eights by the same name."

Newton had been a little more experimental with musical scales in his undergraduate days, toying with them as mathematical games. A short essay “Of Musick”, written around 1665, listed all possible combinations of the five tones and two half-tones making up an octave. The scales, equivalent to white keys on a piano, were rewritten with note names in the same College Notebook. A table entitled, “A Catalogue of ye 12 Musicall modes in theire order of gratefulnesse”, placed G at the top. (Note that it had an F natural, not the F sharp of the modern key of G.) The D mode of “Opticks” was ranked second, of less ‘sweetnesse’, since one semitone fell slightly closer to a keynote. (Other modes on C, A, and E followed in order, all having semitones next to the keynote and/or the fifth. F, with an imperfect 4th was last; B did not rank at all, due to an imperfect 5th.) But, almost fifty years later, Newton would elevate his second-ranked D mode to exemplify colour music, due to its convenient symmetry.

Newton adapted Descartes’ diagram of 1650 (Illustration 4 above), converting the three hexachords on G, C and F into seven-note scales. Two others were added in the inner rings, on A and D, and all span one octave. Around the rim, measures of intervals are given in fractions of the octave, as divided into 53 or 12 parts. As well as the usual black note B flat, there are three additional accidentals – E flat plus F and C sharps. Newton's sketch outlines the music of his day, as it transitions from a modal system to the tonality of keys and scales.

Musical attention was shifting to the octave - an eight-note interval larger than the six-note hexachord – and the ways it might be internally divided. A dozen octave scales were scattered throughout Newton’s College Notebook, all on G, and described equally as a ‘key’ or a ‘mode’. Their notes were given letters rather than sol-fa names, and separated by intervals calculated in various ways. The just intervals - a 16 : 15 semitone, major tone of 9 : 8, and 10 : 9 minor tone– were the gold standard for Newton and many other theorists. He also explored microtones, tiny equal fractions of an octave that could be combined to approximate any kind of interval. After trying a dozen divisions, Newton found 1/53 of an octave the most satisfactory unit. (Perhaps he was following the suggestion of Boethius, who had ascribed the same division to the ancient Greek musician Philolaus.) Adapting them to just intonation gave a semitone of 5 units, and major and minor tones of 9 and 8 units each. Around the same time, Nicolaus Mercator took extensive notes on dividing the octave, frequently using logarithms. He, too, noted the 1/53 fraction, calling it an “artificial comma”. Later in the 17th century, Christiaan Huygens and others would prefer 31 microtones per octave, for use with meantone temperament. Intended for instruments with fixed tuning, the system required keyboards with movable sets of strings, or split keys to activate separate strings. A few such elaborate devices were built, from the 1550s on, but the gap between the mathematical ideal and its implementation became apparent. Nevertheless, Huygen's '31-tet' still has its adherents today.

With the advent of electronic music, some keyboards can reproduce many of the historical tunings at the press of a button. But the most commonly accepted tuning today is equal tempering, a division of the octave into twelve identical semitones. Some modern composers have developed unique microtonal methods, in the hope of escaping the imperfect rigidities of equal temperament. Newton shared their distaste, at least in theory, though his early notes reveal he had tried twelve equal divisions of the scale. But, for most armchair musicians of the 17th century, its notes were far from ideal. Much finer distinctions within the octave were made easily with the use of logarithms. Overall, Newton thoroughly explored the musical issues of his day. He approached the subject academically, as a mathematical exercise (albeit with a philosophical dimension) rather than as a creative endeavour in its own right.

So what made Isaac Newton persevere with colour music? There was no hint of the subject for the first 125 pages of "Opticks", which dealt mainly with effects and properties of colour produced by prisms and lenses. With little warning, the spectrum was given its first musical makeover in Experiment 7, of Book 1, Part 2. In the Definition immediately before it, Newton changed gears; up till then he had been speaking "not philosophically and properly, but grossly", for the benefit of "vulgar People". Having been won over, the reader is given the telling insight that light rays are not coloured themselves, but merely "stir up a Sensation of this or that Colour". The senses as we experience them were considered illusory, creations of the mind or impressions on the sensorium. Objects that caused sensations, outside us in the physical world, had no sensual characteristics. The sound of a bell or of a musical string was no more than a trembling of the air; an object had no colour, only a disposition to reflect or refract certain light rays. It was their motion - of trembling air or speeding light particles - that impacted on our sense organs, to create the form of sound or colour in the sensorium. Up to that point, we can only distinguish any order or meaning in light and sound objectively, through physical experiments and mathematical analysis.

Newton suspected the very different stimuli for sight and hearing were converted, by the physiology of the human body, into somewhat similar experiences. In 1682-3, Dr William Briggs had forwarded an explanation of how the eye worked to the Royal Society, as "A new theory of vision". Briggs studied the optic nerve and pondered how its two separate branches, one to each eye, could produce a single image. He concluded that corresponding fibres in each branch were under the same tension, like musical strings of the same pitch. Consequentially, "when any impression from an object without, moves both Fibres, it causes not a double sensation no more than Unisons in two Viols struck together cause a double sound". The musical simile continued: nerve fibres were likened to the strings of lutes as well as viols, either discordant with each other or "as it were symphonical". In correspondence with Briggs, Newton expressed reservations about "every pair of fellow fibres being unisons to one another, discords to ye rest", but later speculated how harmonies and discords of colour might occur by other means. Newton praised Briggs's ingenious Theory, encouraged him to publish the work in Latin, and wrote a preface for it in 1685. There, Newton thanked the author for the benefit he had derived from the latter's anatomical skill, and emphasized the hypothesis of vibrations as an explanation of action along the nerves. In a similar way, the form of a colour or a sound alike could be carried to the sensorium.

According to Query 23, at the end of "Opticks", Book 3, it was aether (or some like substance) that conveyed the motions in vibratory pulses, along the optic and auditory nerves to the sensorium. So colours and sounds, at least in the way they reached the brain, were much the same thing. Query 14 reinforced the similarity: since it was known from the laws of music, that harmony arose when two or more vibrations of the air were in due proportion, a similar process might cause colour harmony. Newton questioned whether proportional vibrations of the aether in the optic nerve might not cause pleasing colour sensations. For the purpose of the generalization he treated colour names as precise locations, like musical notes, rather than broad intervals as described in the rest of the text. He cited gold and indigo together as an example; in an earlier draft for "Opticks", orange and indigo, red and sky-blue, and yellow and violet, were considered harmonious pairs "for they are fifts", while adjacent colours were discordant, as being "but a note or tone above and below". It was in the speculative realms of an hypothesized aether and proportionate harmonies that Newton summarized his analogy between colour and musical sound - not in the field of science as we now know it. Such was the proper procedure, so it seems, for a natural philosopher of the 17th century.