Chromatics, that branch of optics which treats of the mathematical relations of colors. White light may be compared to a full chord in music, containing all the notes in the octave, but the comparison must not be insisted on too closely. By refraction through a prism, the colors may be separated more or less perfectly. This separation of colors by refraction takes place in nature by means of rain drops, producing the parti-colored rainbow, or by means of minute snow crystals, producing halos. By experiment, refraction can be produced in a great variety of ways, and the different colors of white light can also be brought out by other means than refraction. The mean distance between two waves in a ray of light is .0000225 of an inch; in violet-colored rays, .0000167; and in red rays, .0000266. To chromatics also belongs the discussion of the phenomena of polarization and double refraction. These phenomena depend upon the form of the wave of light, and the direction of the motion in that wave, the wave itself always going in a straight line.

It is in these most minute and subtle optical investigations that the undulatory theory of light has proved most fruitful.