COLOR. Color may be studied objectively as a wave phenomenon (see LIGHT), in its psycho-physiological aspect as a retinal stimulus (see VisioN), or, subjectively, as an appearance or ocular sensation ; this last is the viewpoint of the present article.
Isaac Newton ((Philosophical Transac tions,' No. 80, 1672) was the first to show that white light (sunlight or daylight) may be re solved into elemental colored lights or rays which cannot be further decomposed, but which on being superposed form white light. The question whether sunlight contains these colors in the form of a mixture analogous to a mix ture of, say, nitrogen and oxygen gases in the air or is a compound, as water is of hydrogen and oxygen, belongs to physics. Newton adopted the first point of view ; at the present time the second is gaining favor. The elemental lights, distinguished visually by their colors, are seen in the rainbow, the sparkle of a diamond, in soapbubbles, in a film of oil on water, by look ing at one's eyelashes through nearly closed lids, and in many other phenomena discussed in phys ical optics under the names dispersion, Inter ference, absorption, polarization, fluorescence, diffraction, refraction. They are called spec trum colors—popularly, "colors of the rain bow"— and are monochromatic, i.e., they consist of single colors (colored lights or rays). that are elemental in the sense in which this term is used in chemistry. We are accustomed to think of the spectrum as having only six colors: red (R), orange (0), yellow (Y), green (G), blue (B), violet (V) ; Newton named a seventh, indigo, situated between B and V. But the number of colors is enormously great; Aubert ((Physiologie der Netzhaut,) 1865) estimated the solar spectrum to contain about a thousand distinguishable hues, of which, according to Rood ((Modern Chromatics,' 1881) 2,000,000 tints and shades can be distinguished. However, these differences cannot be seen its situ in the spectrum because the color areas are too small. Lucldesh ((Color and its Applications,' 1915) states that 55 distinctly different hues have been seen in a single spectrum.
Color Color is a sensation pro duced through the excitation of the retina by rays of light. Thus black (Bk) and white (W) are color sensations, although Bk is due to no, and W to complete, stimulus. The infra-red rays —below the red end of the spectrum — and the ultra-violet — beyond the violet end — are invisible. Heat is the principal effect of the infra-red and chemical action, as on a photo graphic plate, that of the ultra-violet rays. To the physicist all of these visible and invisible emanations are regarded as electromagnetic. waves differing essentially in wave length alone. All have chemical and thermal effects but only some give to the eye the sensation called color. Although none of the spectral colors can be resolved into others, some can be imitated to the eye by combinations of others. Combina tions of R and V produce purples which are not in the spectrum at all; such compounds are nevertheless resolvable into their constituents by spectroscopy. Two colors not distinguish able to the eye may have quite different com positions. For example, a solution of potassium dichromate and one of potassium dichromate and neodymium ammonium nitrate in suit able proportions impress the eye as having pre cisely the same yellow color; but whereas a mer cury arc light will look yellow on being viewed through the first solution, it will look brilliant green through the second. No pigment is mon ochromatic, yet it is possiblein many cases to match spectral colors by means of pigments. Color to the eye is therefore not color in the sense of being one definite constituent of white light. Since color is entirely subjective it must be influenced by personal idiosyncracy; indeed some people are color-blind and cannot, for in stance, tell red f rom green. (See VisioN). On the other hand, santonin poisoning produces, in otherwise normal people, a pathological condi tion called chromatopsia, in which all "colors" are yellow. In short, the eye is unable to ana lyze color in the way that the ear can detect the component tones in a chord.