Page:A color notation (Munsell).djvu/62

 of its ether wave. ‘These waves have been measured, and science can name each hue by its wave length. ‘Thus a certain red is known as M. 6867, and a certain green sensation is M. 5269. Without attempting any scientific analysis of color, let it be said that Sir Isaac Newton made his series of experiments in 1687, and was privileged to name this color sequence by seven steps which he called red, orange, yellow, green, blue, violet, and indigo. Later a scientist named Fraunhofer discovered fine black lines crossing the solar spectrum, and marked them with letters of the alphabet from a to h. These with the wave length serve to locate every hue and define every step in the sequence. Since Newton’s time it has been proved that only three of the spectral hues are primary; viz., a red, a green, and a violet-blue, while their mixture produces all other gradations. By receiving the spectrum on an opaque screen with fine slits that fit the red and green waves, so that they alone pass through, these two primary hues can be received on mirrors inclined at such an angle as to unite on another screen, where, instead of red or green, the eye sees only yellow.

(90) A similar arrangement of slits and mirrors for the green and violet-blue proves that they unite to make blue, while a third experiment shows that the red and violet-blue can unite to make purple. So yellow, blue-green, and purple are called secondary