If we define as the "dispersion" in a particular part of the spectrum the ratio of the angular interval d[theta] to the corresponding increment of wave-length d[lambda], we may express it by a very simple formula. For the alteration of wave-length entails, at the two limits of a diffracted wave-front, a relative retardation equal to mnd[lambda]. Hence, if a be the width of the diffracted beam, and d[theta] the angle through which the wave-front is turned, Entry: 1