Fact 1263 — Newton and the seven colors of the spectrum

In his early optical lectures, Newton described the spectrum using five principal colors: red, yellow, green, blue, and violet. In *Opticks* (1704), he identified seven named colors and compared their proportions with the divisions of a musical chord. Historians have proposed that the musical analogy may have influenced the addition of orange and indigo, but the reason cannot be stated as certain fact. The spectrum is a continuous range of hues, and its division into seven is one historical naming scheme.