Why is Indigo in the Rainbow?

Описание: Indigo may be a very vague and unnecessary color, but it has an interesting history that involves some plants, turmoil, and Isaac Newton's interest in the number seven.

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Sources:
http://www.npr.org/2011/11/07/1420941...
http://www.theatlantic.com/video/inde...
http://www.popsci.com/blog-network/te...
http://www1.umn.edu/ships/updates/new...
http://www.npr.org/books/titles/14209...
http://www.nytimes.com/2015/11/06/fas...
http://www.nytimes.com/2014/08/07/gar...
http://www.wildcolours.co.uk/html/ind...
http://news.nationalpost.com/news/why...
http://health.howstuffworks.com/menta...
http://science.howstuffworks.com/colo...
https://books.google.com/books?id=Pbs...

Image Sources:
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://en.wikipedia.org/wiki/File:S_...