“Intensity” is a misleading term, if chroma be intended, for it depends on the relative light of spectral hues. It is a degree rather than a quality, as appears in the expressions, intense heat, light, sound,--intensity of stimulus and reaction. Being a degree of many qualities, it should not be used to describe the quality itself. The word becomes especially unfit when used to describe two very different phases of a color,--as its intense illumination, where the chroma is greatly weakened, and the strongest chroma which is found in a much lower value. “Purity” is also to be avoided in speaking of pigments, for not one of our pigments represents a single pure ray of the spectrum. Examples are constantly found of the mental blur caused by such unfortunate terms, and, since misunderstanding becomes impossible with measured degrees of hue, value, and chroma, it seems only a question of time when they will take the place of tint, tone, shade, purity and intensity. CHAPTER II. COLOR QUALITIES. (20) The three color qualities are hue, value, and chroma. +HUE is the name of a color.+ (21) Hue is the quality by which we distinguish one color from another, as a red from a yellow, a green, a blue, or a purple. This names the hue, but does not tell whether it is light or dark, weak or strong,--leaving us in doubt as to its value and its chroma. Science attributes this quality to difference in the LENGTH of ether waves impinging on the retina, which causes the sensation of color. The wave length M. 5269 gives a sensation of green, while M. 6867 gives a sensation of red.[6] [Footnote 6: See Glossary for definitions of Micron, Photometer, Retina, and Red, also for Hue, Tint, Shade, Value, Color Variables, Luminosity, and Chroma.] +VALUE is the light of a color.+ (22) Value is the quality by which we distinguish a light color from a dark one. Color values are loosely called tints and shades, but the terms are frequently misapplied. A tint should be a light value, and a shade should be darker; but the word “shade” has become a general term for any sort of color, so that a shade of yellow may prove to be lighter than a tint of blue. A photometric[7] scale of value places all colors in relation to the extremes of white and black, but cannot describe their hue or their chroma. Science describes this quality as due to difference in the HEIGHT or amplitude of ether waves impinging on the retina. Small amplitudes of the wave lengths given in paragraph 21 produce the sensation of dark green and dark red: larger amplitudes give the sensation of lighter green and lighter red. [Footnote 7: See Photometer in paragraph 65.] +CHROMA is the strength of a color.+