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In page Indigo:

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The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet.[citation needed] 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."[9]