V. On intensity relations in the spectrum of helium

The two most fundamental characteristics of a spectrum line are its wave-length and its intensity, and it is very remarkable that, at the present time, while the former can often be expressed with an accuracy of one part in half a million, the tabulated intensity may frequently be affected by an error even greater than a thousand percent. Yet for the elucidation of the main problems of astrophysics the relative intensities of spectrum lines may assume an importance scarcely inferior to that of a precise knowledge of their wave-lengths. Although data of the latter kind afford precise evidence of the presence of certain elements, and of the motions of stars and nebulae in the line of sight, it is to the distribution of energy in the spectrum and to the reproduction of specified conditions in the laboratory that we must look for a further knowledge of the physical and more especially the electrical conditions obtaining in celestial bodies. The changes which occur in spectra under varying conditions of excitation are often of a very conspicuous character, and the study of “spark” or enhanced lines has already led to results of fundamental importance, but the observation of such phenomena depends for its success upon the magnitude of the changes involved, and whereas the appearance of new series of lines under appropriate conditions is often apparent at once, a strictly quantitative determination of the relative intensities of the spectrum lines is necessary for the study of the less conspicuous changes, which may, nevertheless, be of fundamental importance. In particular, the intensity changes occurring under varying conditions in lines belonging to the same or to mathematically related series must be a matter for serious consideration in any theory of radiation which involves a theoretical interpretation of the laws of spectra.