Indigo - Classification As A Spectral Color

Classification As A Spectral Color

It was Isaac Newton who first introduced indigo as a spectral color. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company was beginning to import indigo dye into England, supplanting the homegrown source of blue dye woad. In one of the pivotal experiments 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 he acknowledged that the spectrum had a continuum of colors, but specifically named seven colors: "The originall or primary colours are Red, yellow, Green, Blue, & a violet purple; together with Orange, Indico, & an indefinite varietie of intemediate gradations." He linked the seven prismatic colors to the seven notes of a western major scale, as can be seen in his color wheel, with orange and indigo providing the semitones, thinking that perhaps perception of the two was related in some way. 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 accordingly counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonic Roy G. Biv. Maxwell and von Helmholz accepted indigo as an appropriate name for the color flanking violet in the spectrum.

It has been suggested that Newton was naming the colors differently to the present day understandings of them: As Gary Waldman writes: "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." If this is true, Newton's seven spectral colors would have been:

The human eye is relatively insensitive to hue changes in the wavelengths between blue and violet, and if this is where Newton meant indigo to be, most individuals find it hard to distinguish. As Isaac Asimov noted, '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.' Color scientists typically divide the spectrum at about 450 nm between violet and blue, with no indigo.