Why is Indigo in the Rainbow?

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/2011/11/07/142094103/indigo-the-indelible-color-that-ruled-the-world
http://www.theatlantic.com/video/index/283432/why-roygbiv-is-arbitrary/
http://www.popsci.com/blog-network/techtiles/problem-indigo
http://www1.umn.edu/ships/updates/newton1.htm
http://www.npr.org/books/titles/142094203/indigo-in-search-of-the-color-that-seduced-the-world#excerpt


http://www.wildcolours.co.uk/html/indigo_history.html
Why the colour indigo is disappearing from Sir Isaac Newton’s ‘occult’ rainbow
http://health.howstuffworks.com/mental-health/human-nature/perception/eye3.htm
http://science.howstuffworks.com/colors-in-visible-light-spectrum.htm
https://books.google.com/books?id=PbsoAXWbnr4C&pg=PA193&dq=Newton+color+Indigo&hl=en&ei=kw3QTemzJsea8QOC_anbDQ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CH0Q6AEwBg#v=onepage&q=Newton%20color%20Indigo&f=false

Image Sources:
https://commons.wikimedia.org/wiki/File:Indigofera_tinctoria1.jpg
https://commons.wikimedia.org/wiki/File:COLLECTIE_TROPENMUSEUM_Indigo_wordt_aangemaakt_in_kuilen_in_uitgehakte_puimsteentufgrond_Karolanden_TMnr_10014190.jpg
https://commons.wikimedia.org/wiki/File:Indigo_cake.jpg
https://commons.wikimedia.org/wiki/File:Indigo_plant_extract_sample.jpg
https://commons.wikimedia.org/wiki/File:Indo-Roman_trade.jpg
https://commons.wikimedia.org/wiki/File:Indigoproduktion_BASF_1890.JPG
https://commons.wikimedia.org/wiki/File:GodfreyKneller-IsaacNewton-1689.jpg
https://commons.wikimedia.org/wiki/File:NewtonDualPrismExperiment.jpg
https://commons.wikimedia.org/wiki/File:Newton%27s_colour_circle.png
https://en.wikipedia.org/wiki/File:S_Club_7_-_Bournemouth_2015.jpg