s. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full
octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral
purple colors are observed when red and violet light are mixed. In the 13th century,
Roger Bacon theorized that
rainbows were produced by a similar process to the passage of light through glass or crystal. In the 17th century,
Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book
Opticks. He was the first to use the word
spectrum (
Latin for "appearance" or "apparition") in this sense in print in 1671 in describing his
experiments in
optics. Newton observed that, when a narrow beam of
sunlight strikes the face of a glass
prism at an angle, some is
reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light is bent (
refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors. 1855) Newton originally divided the spectrum into six named colors:
red,
orange,
yellow,
green,
blue, and
violet. He later added
indigo as the seventh color since he believed that seven was a perfect number as 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. 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 with a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas his "blue" corresponds to
cyan. In the 18th century,
Johann Wolfgang von 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. 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. 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. ==Limits to visible range==