§ 1. IT was proved by Gouy and Lord Rayleigh that interference may be observed by the aid of the spectroscope even if the incident light were entirely irregular, consisting, for instance, of an irregular succession of impulses. This means that the “regularity” of the emergent light arising from an impulse is due to the spectroscope itself, and the extent to which interference may be observed is limited only by the resolving power of the instrument used. The greater the resolving power, the more homogeneous is the light in any part of the spectrum. The question regarding the modus operandi of the prism has been further dealt with by Professor Schuster in his paper “On Interference Phenomena” (Phil. Mag., vol. xxxvii., 1894) and, later, in his explanation of Talbot's bands by consideration of group-velocities (Phil. Mag., vol. vii., 1904).