Violet (i.e. reddish blue) is visible in our spectrum as well as Newton’s, and Newton certainly did not add it to bring the number of colours to seven; because it is in both the five-hue and the ten-hue divisions of the spectrum he described in his Optical Lectures of 1670-72. Dr Burton may be getting confused with the suggestion that has often been made that Newton included indigo in the final seven to fit with his suggested analogy with a musical scale.
Also, Newton did not give “each of the color as much proportional room on the wheel as they have in a spectrum”, but instead adjusted the intervals to those that the notes of a Dorian modal scale occupy in a circular diagram. See Fig. 7.1.7 and 7.1.8 here:
http://www.huevaluechroma.com/071.php

Looking forward to reading more and seeing more of your stuff!

Mike

This page http://en.wikipedia.org/wiki/Lab_color_space has some images that give an idea of what that space (including lightness) is like. Purple is definitely there between blue and red. L*a*b* models colors as our brains perceive them in such a way that distances in the space match amount of difference between colors as people report perceiving it. I think it’s Munsell who first systematically built a color model based on that kind of experiment.

(L*a*b* also suggests why orange seems to deserve a place of its own, whereas RGB goes red, yellow, green..)

I have to admit the existence of purple always bothered me and I sort of took people’s word for it that the colors indigo and violet existed at the end of the rainbow.

Physical color is *not* one-dimensional! Light is a mixture of different amounts of *all* the possible wavelengths in the range we can perceive. Each wavelength amounts to a separate dimension, so the physical color space is infinite-dimensional (although a finite amount of energy can only convey a finite amount of information). That infinite-dimensional variety of colors is reduced to four dimensions by the RGBW sensors in the eyes, and then down to three dimensions by the retina’s coding scheme, then down to two dimensions if you’re ignoring lightness, and then a loop if you’re looking for colors as opposed to gray.

The R, G and B sensors each respond to a different extent to each spectrum line, their responses form three funky overlapping curves http://en.wikipedia.org/wiki/Color_vision#mediaviewer/File:Cone-fundamentals-with-srgb-spectrum.svg . After encoding to R-G and B-Y, the responses to individual spectrum lines arrange themselves in an unevenly-spaced way on a nearly-closed C curve around that perceptual room.

(The red and green response curves are very similar, but evolution apparently thinks that that difference is very important, and that’s why the yellow space gets stretched enough for orange to have a place.)

I guess you could say we *confuse* the spectrum with the loop because of that C curve. Maybe that’s why people imagine indigo and violet there. Moving along the C curve, you’re moving in the direction (in the room) towards purple as the blue fades out. The idea that purple is on the rainbow is like the idea that the end of a rainbow touches down somewhere on the earth (which I only clearly realized in adulthood that it doesn’t). The earth and purple are really there, but the end of the rainbow just perceptually points *toward* purple the way it points toward the earth.

Between nought and one on the line we can make divisions two halves four quarters etc. Try and dig a hole in the ground and you will find a new level which is not negative?