Investigation of stellar spectra has been active during the last four years. Without attempting to make a complete survey, some important researches may be mentioned.
The theory of the intensities and contours of absorption lines has been discussed by Eddington, Milne, Pannekoek, Woolley, H. H. Plaskett, and others, and has proved to be difficult. For example, no quantitative theory has yet been developed for the residuai intensity which remains in even the strongest lines, although the physical causes underlying the formation of residual intensities are rather obvious. Study of solar lines with light which has left the surface at different angles shows that the processes which produce the wings and the centre are probably different. The most promising line of attack on this intricate problem appears to be in studies of the solar spectrum, where different regions of the disc may be separately investigated, rather than of integrated starlight. The highest dispersion available, even in solar work, will be none too great.