The Structure of Interstellar H and K Lines in Fifty Stars.

The structure of the interstellar H and K lines of Ca ii appearing in the spectra of 50 stars of early type has been studied in the second order of the grating spectrograph at the coudé focus of the 100-inch telescope. The camera has a focal length of 114 inches and gives a linear scale of 2.9 A/nim. More than 80 per cent of the stars show complex lines consisting of two to five components. Stars in various regions show differences in complexity, those in Perseus and Scorpius as a rule having single lines or at most lines with a faint component, while those in Orion, Sagittarius, and Cygnus usually show more intricate structure. The intensities of the components have been estimated. Measurements of their radial velocities provide a means of distinguishing individual interstellar clouds in various portions of the sky. Many of the clouds are found to extend over great areas. The narrowness of the components affords strong evidence for the discrete character of the clouds and the absence of any considerable tur- bulence. The spectra have been examined for the additional narrow interstellar lines discovered and identified during recent years, and their intensities have been estimated. Lines of Ca i and Fe i seem to appear only in stars which show very strong H and K lines, while those of the diatomic molecules CN, CH i, and CH ii may occur when H and K are relatively weak. The lines of CH i and CHii showed marked differences of intensity in various stars. The radial velocities from some of these additional lines in a few selected stars have been compared with the velocities from the components of H and K in the same stars. In this way it has been possible to distinguish the interstellar clouds in which the various lines are formed