<P> Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet . He chose seven colors out of a belief, derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week . The human eye is relatively insensitive to indigo's frequencies, and some people who have otherwise - good vision cannot distinguish indigo from blue and violet . For this reason, some later commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet . However, the evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words . Comparing Newton's observation of prismatic colors to a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas "blue" corresponds to cyan . </P> <P> In the 18th century, Goethe wrote about optical spectra in his Theory of Colours . Goethe used the word spectrum (Spektrum) to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors . Goethe argued that the continuous spectrum was a compound phenomenon . Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish - yellow and blue - cyan edges with white between them . The spectrum appears only when these edges are close enough to overlap . </P> <P> In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet), Thomas Young, Thomas Johann Seebeck, and others . Young was the first to measure the wavelengths of different colors of light, in 1802 . </P> <P> The connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century . Their theory of color vision correctly proposed that the eye uses three distinct receptors to perceive color . </P>

Human experience the longest visible electromagnetic waves as