Newton's Color theory

Fig 3 Newton's original color spectrum showing 7 colours in sequence, Red (p), Orange (q), Yellow (r), Green (s), Cyan (t), Ultramarine (v) and Violet (x).5

Newton was drawn to the harmonies achieved by music and thought that the color spectrum 'should' replicate such harmonies. He then based his new 7-color spectrum on a musical scale, which had 7 notes including half notes or accidentals. To account for the accidentals he added two more colours, Orange and Ultramarine. The spectrum in fact represents an arbitrary division of the colours simply shaped by Newton's appreciation of harmonious music

Newton would later re-name the colours to the conventional ROYGBIV. Black and white have been excluded, and instead all sections meet at a vacant center symbolizing their combination to be white light. The letters on the border of each section are the musical scale equivalent, thus the two smaller sections, Orange and Indigo, are bordered by E-F and B-C respectively represent the accidentals. The circular figures in each color section show two things about what Newton believed of the nature of light. The size of the circles get progressively smaller as the wave length of the colours do, indicating first that Newton believed light to be corpuscles, shown by the choice of symbol, and secondly that the corpuscles were deflected by the prism according to their size, Red being the largest circle(5). Unless we accept Newton's appreciation for harmony and his belief that music and color are one and the same then his color circle remains equally as arbitrary as the musical scales themselves. This suggests that perhaps the 7 primary colours have more of an aesthetic value than a scientific value.

In De-mystifying De Clario, author Niels Hutchinson presents interesting explanations for the choice of Indigo and Orange as names for the two extra colours in the circle. For Newton, Indigo would have been a commonly known dye, used because of its ability to permanently dye all kinds of fibers. It was further made important by the attempted monopolization of the British and Dutch East India companies and it remained in use around the world for hundreds of years. Hutchinson professes that both a lighter Blue (Cyan) and a more royal Blue (Indigo) can be seen in the refractions of sunlight through cut glass or prisms. One suggestion for Newtons inclusion of Indigo in the rainbow is that Newton wanted to acknowledge the existence of two different blues, although this difference is not recognizable in a typical rainbow (see Figure 1). An explanation for the inclusion of Orange is that Newton wished to honor a close patron, William III, Duke of Orange, who passed away two years before Optiks was published. These two explanations provide another insight into why he would include Orange and Indigo as the 6th and 7th colours of the rainbow, further questioning the scientific value of his color circle(4).

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