Bakerian lecture.— Diffuse matter in interstellar space

Source abstract

Abstract 1. The title of this lecture naturally provokes the question, Is there any appreciable quantity of matter in ordinary regions of space between the stars ? I admit that it is rather an important question, and I should scarcely feel justified in devoting a Bakerian lecture to the subject if I did not think the answer was in the affirmative. But from the narrower astronomical point of view it is not a question to be insisted on at the outset. Astronomers are, in fact, so placed that they must presume the existence of such matter unless or until its absence is proved. For many years interstellar matter has figured in astronomical investigations, but chiefly with a negative importance. Three examples may be given— (1) In determining spectroscopic parallaxes and in gauging the distances of clusters and spiral nebulae by means of Cepheid variables it is assumed that there is no absorption or scattering of light by interstellar matter.