I read recently that the only reason that we say there are seven colours in the rainbow is because newton decided there should be as many colours as there are days in the week.
I'd be interested to know what you were reading. I've never encountered that claim before, but it certainly sounds like something Newton would do. He had a head full of mysticism, spent a good deal of his later years mucking about with magical stuff. He wasn't so much the first of the great scientific (in the modern sense) thinkers as he was the bridge between magical and scientific thinking. I've always wondered about the spectrum too, it's never appeared to have 7 colours to me. Red, orange, yellow, green, blue, and violet I can readily make out, but that supposed indigo stripe between blue and violet is a bit dodgy.
