218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 3 



factory. While the tendency among astronomers now is to favor the 

 hypothesis of a medium of interstellar dust, which is not coexistent 

 with the cloud of interstellar gas giving rise to stationary absorp- 

 tion lines, we must admit that much work remains to be done, both 

 observationally and theoretically, before we shall be able to definitely 

 accept this interesting hypothesis. 



To sum up these recent results of astronomical investigation: 

 Space is no longer to be conceived of as completely empty in those 

 regions not occupied by the individual stars and the bright nebulae. 

 The whole intervening regions are populated by obscuring gases so 

 very rare, it is true, as to be far beyond our best vacuums, but still 

 probably containing about the same mixture of chemical elements 

 that we find in the sun or the earth's crust. Besides these gases 

 astronomers are now generally convinced that there is also in space 

 a very rare dusty haze made up of material particles. It is indeed 

 so very rare a haze that only by refined measurements does its effect 

 of obscuring the stars become manifest. It tends to make the dis- 

 tant stars more reddish. 



