page_title: Indigo
section_title: Classification as a spectral color
hierarchical_section_title: Indigo / Classification as a spectral color
caption_attribution_description: English: "In a mixture of primary colours, the quantity and quality of each being given, to know the colour of the compound." Throughout Opticks, Newton compared colours in the spectrum to a run of musical notes. To this purpose, he used a Dorian mode, similar to a white-note scale on the piano, starting at D. He divided his colour wheel in musical proportions round the circumference, in the arcs from DE to CD. Each segment was given a spectral colour, starting from red at DE, through orange, yellow, green, blew [sic], indigo, to violet in CD. (The colours are commonly known as ROY G BIV.) The middle of the colours—their 'centres of gravity'—are shown by p, q, r, s, t, u, and x. The centre of the circle, at O, was presumed to be white. Newton went on to describe how a non-spectral colour, such as z, could be described by its distance from O and the corresponding spectral colour, Y. A higher resolution image of this would be nice, if someone has access to one.
context_page_description: Indigo is a deep and rich color close to the color wheel blue, as well as to some variants of ultramarine, based on the ancient dye of the same name. The word indigo comes from the Latin for "Indian", as the dye was originally imported to Europe from India.
  It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum.
  The first known recorded use of indigo as a color name in English was in 1289.
context_section_description: The Early Modern English word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic color-naming system. Modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers, which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet.
  However, the correspondence of this definition with colors of actual indigo dyes is disputed. Optical scientists Hardy and Perrin list indigo as between 445 and 464 nm wavelength, which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.
  Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England, supplanting the homegrown woad as source of blue dye. In a pivotal experiment in the history of optics, 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." He linked the seven prismatic colors to the seven notes of a western major scale, as shown in his color wheel, with orange and indigo as the semitones. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:
  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.
  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.
  Later scientists conclude that Newton named the colors differently from current usage.
  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, cyan or light blue." If this is true, Newton's seven spectral colors would have been:
  The human eye does not readily differentiate hues in the wavelengths between what we today call blue and violet. 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."
  Modern color scientists typically divide the spectrum between violet and blue at about 450 nm, with no indigo.
caption: Indigo is one of the colors on Newton's color wheel
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