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From: moroney@world.std.spaamtrap.com (Michael Moroney)
Newsgroups: sci.physics
Subject: Re: Are English school children really taught that there are seven colours in the visible spectrum?
Date: Fri, 24 Jun 2016 18:42:21 +0000 (UTC)
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Peter Percival <peterxpercival@hotmail.com> wrote:
> I know that Newton pretended that there were seven colours in the 
> rainbow (presumably he didn't really believe it).

The Wikipedia rainbow article https://en.wikipedia.org/wiki/Rainbow states
Newton initially used five colors but later added orange and indigo because
the number seven was considered special (musical notes, (known) objects in
solar system, days of week) at the time.