The colors of the rainbowNewton's ideas on color were not correct, in the modern sense. He placed seven colors on a wheel, allocating space in relation to the Dorian music scale (that he had described) with seven notes. Newton's seven colors are not seen in a normal large raindrop rainbow, which has five, (RYGBV), and they are not seen in the spectrum produced by refracting sunlight through a glass prism. Average eyes do not allocate his sizes to the segments on his color wheel on the basis of the length of the color patches seen in a linear strip. Add to this is the difficulty of photographing the spectrum in true color and the fact that the appearance of a rainbow depends on droplet size, and we have a confused situation avoided in all elementary texts. Amy Smith has written an essay which clarifies these points - by using a published simulation program - to examine a range of rainbows. Her essay is in the Essays index. In spite of their prevalence, modern textbook illustrations showing ROYGBIV on a line, with equal length allocations, have no basis in reality.
Original at ... http://www.colorsystem.com
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Newton's Optics
Using modern lettering and his original script, we can see that Newton's colour circle comprises seven colours in the sequence red (p) - orange (q) - yellow (r) - green (s) - cyan (t) - ultramarine (v) - violet (x). Black and white have been excluded, and the vacant centre of the circle has instead been expressly assigned to white in order to symbolise that the sum of all the specified colours will result in white light.
In 1648, Marci had directed white light through a prism and observed its deflections. Newton took this a step further, becoming convinced that the deflected light rays ran on in a straight line after passing through the prism. In his "experimentum crucis", Newton directed the rays which had been refracted by a first prism through a second prism. He observed that they were deflected once more, but were otherwise not altered (further separated into colours). For Newton, this was proof that colours are not modifications of white light, but are the original components of white light. White light is composed of coloured light: in fact (according to Newton), the seven colours which are located within the colour-circle. This coloured light is not a mixture. It is a single colour, and is pure. It can be mixed, of course, to produce secondary colours, but if the components combine in the correct proportions, the light will appear white.
The palette formed through the refraction of light by a prism is referred to as the colour spectrum, the components of which are the spectral colours. The question now is, how do we explain them? For what reason is blue light deflected (refracted) in a prism to a greater degree than red light? An answer could only be provided if more was known about the nature of light. What actually was a light ray, which evidently moved in a straight line? Did it involve a wave, as could be seen running along a rope? Or did light comprise tiny particles (corpuscles)?
Newton attempted to clarify these questions in his second definitive work, his Opticks, which first appeared in 1704 and contained the colour-circle which we reproduce. The colours are marked here by circular figures, at their largest for red and becoming progressively smaller towards violet. In this way, Newton reveals something of his ideas about the nature of light. He believed that light was composed of corpuscles which were deflected by a prism according to their size: the large red was subjected to the least deflection, and the small blue the most.
Colours are allocated to segments, the sizes of which are proportional to their respective colour's intensity in the spectrum. Using this segment size, and the varying sizes of the light corpuscles, it was possible to calculate a type of concentration point for the circle - marked as Z by Newton - and mark it in. The straight line, which connected the white colour centre O and this centre of concentration Z, intercepted the circle at Y. Bearing in mind Newton's love of mechanics, colour mixtures can then be represented by drawing a triangle of forces, the three corners of which are formed by the three basic colours: rosso (red), blu (blue) and verde (green).
Newton believed that the propagation of both light and sound to be comparable; they should therefore be treated harmonically in an identical way. Newton selected seven colours because his octave was made up of seven sound intervals. He allocated segments to them in accordance to their value in the Dorian musical scale. The individual sound tones associated with this scale coincide with the borders between the colour grades: D, for example, with the border between violet and red; A with the border between green and blue. This mathematical-musical appropriation of colours makes it difficult for many to understand Newton's system which, with its seven (instead of five) primary colours, has more of an aesthetic basis than a scientific one.
With Newton's colour circle, the transition between the one- and two-dimensional colour system is complete. It is helpful to realise that although this step was made by a physicist, it actually has little to do with physics. The spectrum which Newton sees on the other side of his prism is a line which he can only transform into a circle because the colour tones merge into one another gradually. For this reason alone, the short-wave end (violet) can be joined onto the long-wave end (red). This omission in physics is overcome by our senses: out of the straight line of physics, it is actually the human brain which creates this circle, first drawn by Newton. We understand colours only when we also take into account those who see them.