Editing Indigo (section)

=== Isaac Newton's classification of indigo as a spectral color ===
[[File:Newton's colour circle.png|thumb|class=skin-invert-image|Indigo is one of the colors on Newton's [[color wheel]].]]

Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of [[Dispersive prism|prisms]] at a fair near [[Cambridge]], the [[East India Company]] had begun importing indigo dye into England,<ref>{{cite book|last=Allen|first=O.N. Allen & Ethel K.|title=The Leguminosae: a source book of characteristics, uses, and nodulation|year=1981|publisher=University of Wisconsin Press|location=Madison, Wisc.|isbn=978-0-299-08400-4|page=343|edition=null}}</ref> supplanting the homegrown [[woad]] as source of blue dye. In a pivotal experiment in the [[history of optics]], [[Isaac Newton's early life and achievements#Newton's theory of colour|the young Newton]] shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this [[optical spectrum]], Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."<!--Use the archaic spellings (blew, orang, indico, ...) from the original text per WP:SPELLING--><ref>Newton's draft of ''[http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 A Theory Concerning Light and Colors] {{Webarchive|url=https://web.archive.org/web/20110721021035/http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 |date=21 July 2011 }}'' on newtonproject.sussex.ac.uk</ref> He linked the seven prismatic colors to the seven notes of a western [[major scale]],<ref>{{cite web |url=http://www1.umn.edu/ships/updates/newton1.htm |title=SHiPS Resource Center &#124;&#124; Newton's Colors |access-date=16 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20140929225102/http://www1.umn.edu/ships/updates/newton1.htm |archive-date=29 September 2014 }}</ref> as shown in his color wheel, with orange and indigo as the [[semitone]]s. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:
[[File:Newton prismatic colours.JPG|thumb|class=skin-invert-image|upright=1.59|Newton's observation of prismatic colors: Comparing this to a color image of the visible light spectrum shows that "blue" corresponds to [[cyan]], while "indigo" corresponds to [[blue]].]]
<blockquote>I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.<ref>{{cite book|last=Brewster|first=David|title=Memoirs of the life, writings and discoveries of Sir Isaac Newton, Volume 1|year=1855|page=408|url=https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145344/https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|url-status=live}}</ref></blockquote>
[[File:Rainbow-diagram-ROYGBIV.svg|thumb|upright=0.86|The traditional seven colors of the rainbow]]
Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonics "Richard of York gave battle in vain" and ''[[Roy G. Biv]]''. [[James Clerk Maxwell]] and [[Hermann von Helmholtz]] accepted indigo as an appropriate name for the color flanking violet in the spectrum.<ref name="ronchi">{{cite book|last=Ronchi|first=Lucia R.|title=The Excentric Blue. An Abridged Historical Review|year=2009|publisher=Fondazione Giorgio Ronchi|isbn=978-88-88649-19-1|url=https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell+and+von+Helmholtz+accepted+indigo&pg=PA51|author2=Jodi Sandford|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145347/https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell%20and%20von%20Helmholtz%20accepted%20indigo&pg=PA51|url-status=live}}</ref>

Later scientists concluded that Newton named the colors differently from current usage.<ref>{{cite book|last=Evans|first=Ralph M.|title=The perception of color|year=1974|publisher=Wiley-Interscience|location=New York|isbn=978-0-471-24785-2|edition=null}}</ref><ref>{{cite journal|last=McLaren|first=K.|title=Newton's indigo|journal=Color Research & Application|date=March 2007|volume=10|issue=4|pages=225–∠229|doi=10.1002/col.5080100411}}</ref>
According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name [[blue-green]] or [[cyan]]."<ref name=Waldman2002>{{cite book|last=Waldman|first=Gary|title=Introduction to light : the physics of light, vision, and color|date=1983|page=193|edition=2002 Dover revised|publisher=Dover Publications|location=Mineola|isbn=978-0-486-42118-6|url=https://books.google.com/books?id=PbsoAXWbnr4C&q=Newton+color+Indigo&pg=PA193}}</ref> If this is true, Newton's seven spectral colors would have been:
{{center|[[Red]]:{{color box|#ff0000}} [[Orange (colour)|Orange]]:{{color box|#ff8000}} [[Yellow]]:{{color box|#ffff00}} [[Green]]:{{color box|#00ff00}} [[Blue]]:{{color box|#54baff}} Indigo:{{color box|#0000ff}} [[Violet (color)|Violet]]:{{color box|#8B00FF}}}}

The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet.{{citation needed|date=March 2025}} If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to [[Isaac Asimov]], "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes, it seems merely deep blue."<ref>{{cite book|last=Asimov|first=Isaac|title=Eyes on the universe : a history of the telescope|year=1975|publisher=Houghton Mifflin|location=Boston|isbn=978-0-395-20716-1|page=[https://archive.org/details/eyesonuniverse00isaa/page/59 59]|url-access=registration|url=https://archive.org/details/eyesonuniverse00isaa/page/59}}</ref>