ON May 13, 1938, Prof. Charles Fabry delivered the George Darwin Lecture at the Royal Astronomical Society and dealt with the problem of the nature of the matter which exists in interstellar space (Men. Not. Boy. Astro. Soc., 98, 9, Supp. No., October 1938). The subject is so vast that it was possible for Prof. Fabry only to review briefly the different methods of investigation and the interpretation which has been placed upon what they reveal. As is well known, the Doppler effect has enabled the interstellar lines to be detected. The displacement of the lines in stellar spectra gives the radial velocity and this should be the same for all lines. If a line gives a velocity which differs from that derived from the displacements of other lines, the discrepancy must be due to matter situated between the star and ourselves—in other words, interstellar matter. In 1904, Hartmann observed the spectrum of δ Orionis, a star of variable radial velocity, and found that the velocity, measured on the hydrogen lines, varied between +133 and -66 km./sec., but the lines of ionized calcium showed a constant velocity of + 16 km./sec. This supplied the evidence for the existence of a cloud of ionized calcium between the star and ourselves. It seems remarkable that for fifteen years these ionized calcium lines were the only ones which were known with any certainty. In 1919, however, Miss Heger established without doubt the existence of neutral lines of sodium, and with improvements in spectroscopic methods more interstellar lines have been discovered. It is interesting to notice that the exploration of the ultra-violet lines of sodium by Adams and Dunham has been rendered possible by the use of amminized mirrors and of gratings ruled on an aluminized layer. The discovery of other lines which cannot be identified has caused some speculation regarding their origin, and Prof. Fabry conjectures that microscopic crystals, perhaps giving absorption bands at the very low temperature, may be responsible for some of them.
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Interstellar Space. Nature 143, 253–254 (1939). https://doi.org/10.1038/143253a0