142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



If, at the turn of the century, a layman or even an astronomer 

 had been asked what lies in the vast spaces between the stars he 

 M^ould probably have answered, "Little or nothing." There might 

 be an occasional wandering mass of cold rock like an asteroid or a 

 meteorite or specks of dust such as produce our "shooting stars" 

 when they strike the earth's atmosphere; but in general, space was 

 considered as essentially empty, with practically all the material in 

 our galaxy condensed into the stars. 



About 1900 several observations raised serious questions regarding 

 the supposed emptiness of space. The most important of these were 

 Barnard's photographs of the Milky Way which showed great lanes 

 and holelike structures in the huge clouds of stars which compose 

 this shining ring of light. To interpret these as real vacancies where 

 no stars exist was the natural impulse, but gradually observations 

 accumulated which made it impossible to retain this view. The 

 "holes" were too sharply bounded and in many cases were associated 

 with visible cloudlike luminosity which veiled the region. Moreover 

 the presence of numerous long "tunnels" among the stars pointing 

 toward the earth seemed altogether improbable. The final evidence 

 was afforded by the photographs made at the Lick Observatory of 

 the outer universes of stars, the extragalactic nebulae, many of which 

 showed definite streaks of absorbing material crossing the main body 

 of the nebula. The apparent vacancies were due to the presence of 

 dark clouds of cosmic dust which absorb and scatter the light from 

 the stars behind, either obliterating them completely or leaving 

 them comparatively faint and inconspicuous. 



These cosmic clouds are composed of very finely divided particles 

 of dust and are often of enormous extent, especially in the region 

 of the Milky Way. When their thickness is great they blot out the 

 stars behind them, and when thin they redden the starlight passing 

 through them just as dust or smoke in the earth's atmosphere red- 

 dens sunlight, especially near sunrise or sunset when the path through 

 the dust is long. The importance of these cosmic clouds in astron- 

 omy is very great: they affect the brightness and color of every star 

 whose light passes through them, and calculations of the distances 

 of remote stars, the size of our universe, and the quantity of matter 

 within it are all profoundly influenced by the absorption and scatter- 

 ing of light in interstellar space. 



I shall not dwell longer on this most interesting question of dust 

 clouds in space since many of you heard Dr. Scares give a lecture on 

 this subject a few months ago on the occasion of the award to him 

 of the Bruce Medal of the Astronomical Society of the Pacific. To 

 those of you who may not have heard him I can recommend a 

 reading of his admirable presentation of the whole subject in the 



