In the reaction against the arbitrariness of prismatic spectra there seems to be danger that the claim to ascendancy of the so-called diffraction spectrum may be overrated. On this system the rays are spaced so that equal intervals correspond to equal differences of wave-length, and the arrangement possesses indisputably the advantage that it is independent of the properties of any kind of matter. This advantage, however, would not be lost, if instead of the simple wave-length we substituted any function thereof; and the question presents itself whether there is any reason for preferring one form of the function to another.

On behalf of the simple wave-length, it may be said that this is the quantity with which measurements by a grating are immediately concerned, and that a spectrum drawn upon this plan represents the results of experiment in the simplest and most direct manner. But it does not follow that this arrangement is the most instructive.

Some years ago Mr Stoney proposed that spectra should be drawn so that equal intervals correspond to equal differences in the frequency of vibration. On the supposition that the velocity of light in vacuum is the same for all rays, this is equivalent to taking as abscissa the reciprocal of the wave-length instead of the wave-length itself. A spectrum drawn upon this plan has as much (if not more) claim to the title of normal, as the usual diffraction spectrum.