Where did my indigo (part 2)

I answered a question on Quora recently. What is the true color of indigo? There are many answers to this question. In my last post, I gave one of them: indigo is a dye. This blog post expounds on Isaac Newton's answer.

ROYGBIV
We all know the seven colors of the rainbow: ROYGBIV. For those who missed that day in kindergarten, this is an acronym for red, orange, yellow, green, blue, indigo, violet.

But, what slice from the rainbow does indigo get? I consulted my high school optics book, Hardy and Perrin. It told me that indigo is the slice from 446 to 464 nm.

Question answered! Indigo is a very specific slice from the rainbow, which has been scienterrifically defined. 👍

But there is more to the story
But, as you might expect from my blog posts, there is more to the story. I didn't mention that this reference book, Hardy and Perrin, was from 1932. I also didn't mention that I had to dig deep to find this definition in any physics textbook. Perhaps I am being just a tiny bit disingenuous by implying that the definition "446 to 464 nm" is today's scientific consensus, when practically every science textbook I could find neglects to define indigo?

I dug my handy monochromator out of the closet and dialed in 455 nm. This is the location smack dab in the middle of the range that Hardy and Perrin gave for indigo, so that should give me the truest indication of what indigo is.

I looked at it and said, "Oh. It's blue." I asked my wife (who claims to be the most color-literate person who I know) what the color was. She said "blue", and then modified it to "cobalt blue", and then pointed at a lovely flower vase of hers. "Did you notice that there aren't any flowers in my cobalt blue vase?" She smiled and batted her eyes. I'm not sure what she meant by that.

I took a picture of the monochromator output with my cellphone camera. There are lots of caveats here, like RGB cameras don't do a good job at measuring color, and computer monitors don't always produce reliable color, but it kinda looks like my camera is identifying 455 nm light as blue.

Hmmmm... Who had the crazy idea of naming that part of the rainbow indigo? Why not just call it blue?

Newton's rainbow
To answer my rhetorical question, Isaac Newton was the person who had the crazy idea of naming part of the rainbow indigo. In 1665, Isaac Newton took leave from his schooling in Cambridge in order to escape the Great Plague. His work over the next two years proved to be one of the most productive in the history of science. Beyond the whole bit about the inverse square law of gravity, and inventing calculus to do that, Newton made some fundamental observations about light during his sojourn.

He passed sunlight through a prism and demonstrated that, among other things,

       white light is comprised of a whole lot of different flavors of light,

        a prism doesn't impart color to the white light that passes through (as was thought at the time), but rather bends light by different amounts depending on the flavor, and

        these individual flavors could be recombined to make white, or to make a host of other colors if some of the flavors are left out.

This remarkable time period is when he labelled the parts of the rainbow, or rather, the parts of a spectrum projectected on a wall through a prism. The results of his experiments were published by the Royal Society in 1672. These results challenged some long-held notions about light and color... but that's a good topic for another blog post.

Blue vs. blue
This may sound like a change of topic, but bear with me here. In English, there are eleven basic color terms (BCT): white, black, gray, red, orange, yellow, green, blue, pink, brown, and purple. I have written about these before when I tried to identify unambiguous color names. These basic color terms have been the subject of much research since the work of Kay and Berlin.

I offer a quote from a brilliant scholar, one who I respect immensely. I just can't say enough about the guy. Here is the quote from the esteemed John Seymour in his blog post How well do we remember color?

"In some languages, such as Russian, Japanese, and Italian, there is a separate word for light blue which stands on its own as a distinct color."

I have another quote, this one from a guy who is actually a real chromolinguist. Again, I respect him immensely. This is from Dimitris Mylonas

"[Two studies]  found that Russian and Greek languages both have 12 BCTs, differentiating ‘light blue’ from ‘dark blue’."

In Russian, we have two monolexic (single word) names for blue: синий and голубой, which mean "blue" and "sky-blue", respectively. In Japanese, the same concepts are in the words kon and mizu.In Italian, there is blu and azzuro. In Greek, kyaneos refers to dark blue, but it could also mean dark green, violet, black or brown. The ancient Greek word for a light blue is glaukos.

Consider the plight of Isaac Newton when he was trying to assign names to the colors of the rainbow. He looked at this wide expanse of colors which slowly pass from violet to green. If he had been conditioned by being a native speaker of Russian, Japanese, Italian, or Greek, then it may have been obvious to him to call the colors violet, dark blue, light blue, green, and so on.

But Newton spoke English. He did not have basic color terms for the two different types of blue. He had three choices.

1. He could use the terms dark blue and light blue. He probably felt this was kinda dorky. Or at least awkward. Or maybe he just wanted all the color names to be monolexic.

2. He could have chosen blue for the dark blue and found another name for light blue. I don't know what color names were in vogue at the time, but today, he might have used: aqua, aquamarine, azure, baby blue, cerulean, cyan, robin's egg blue, sky blue, teal, or turquoise. But these all strike me as being somewhat ambiguous.

3. He could have instead chosen blue for the light blue, and then found another color name for dark blue.

Newton went with option #3, and chose indigo as the name of dark blue. Here is a quote from Newton:

"So there are two sorts of colours: original and simple colours and colours made by compounding these. The original or primary colours are red, yellow, green, blue, and a violet-purple, together with orange, indigo, and an indefinite variety of intermediate shades."

Why did he pick the word indigo? I have a suggestion that I am dyeing to share. If I may be so bold as to quote the blogger who wrote the first post in this series: "Dutch ships carted nearly 170 tons of the [indigo] dye from India to Europe in 1631." Newton did his work with prisms in 1665. Indigo dye was quite popular in Europe at this time. So (my contention) is that the word indigo was at the time associated with a dark blue dye and that it was a common word at the time.

So, indigo is just another name for blue, only bluer than blue can be.

But, why didn't Newton just call them both blue? Why not just six colors for the rainbow? That is the thrilling question I will answer in my next blog post on this subject!

Errata:
An earlier version of this post omitted green from the list of basic color terms. I apologize to any verdiphiles who were offended. Robin Myers is to be thanked for his ever-vigilant corrections to my blog posts.

Bibliography
Brent Berlin and Paul Kay, Basic Color Terms: Their Universality and Evolution, CSLI Publications, Stanford, California (1999)

Arthur C. Hardy, Arthur C.  and Fred H. Perrin, The Principles of Optics, McGraw-Hill Book Co., Inc., New York. 1932, p. 16
https://archive.org/details/ThePrinciplesOfOptics/page/n29

Dimitris Mylonas and Lindsay MacDonald, Augmenting Basic Colour Terms in English, Color Research and Application, Volume 41, Issue 1, February 2016

Isaac Newton, A New Theory of Light and Colours, Transactions of the Royal Society, 1672

John Seymour, How well do we remember color?, John the Math Guy blog, May 30, 2018
https://johnthemathguy.blogspot.com/2018/05/how-well-do-we-remember-color.html