148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



follow the same pattern ; they may appear anywhere in the nebula but 

 are most frequent in the bright, unresolved nuclear region. Cepheids, 

 on the contrary, follow the complementary distribution of the super- 

 giants; they favor the spiral arms and tend to avoid the nuclear region. 



These notes, which omit many individual features of the triple 

 nebula in Andromeda, and almost the whole of a considerable body of 

 spectrographs data, are yet sufficient for our purpose. The nearest 

 of the neighboring spirals stands forth as a great, independent stellar 

 system, an island universe, separated from our own system by a vast 

 stretch of appreciably empty space. If we were in that nebula, and 

 looked back across that space, the system of the Milky Way presum- 

 ably would resemble the actual appearance of M31 in its dimensions 

 and in many of its structural features. 



As our explorations sweep outward, we recognize countless other 

 members of this same family. They are the true inhabitants of the 

 universe. The nearer systems appear large and bright. Then we 

 find them smaller and fainter in constantly increasing numbers, and 

 we know we are reaching out into space farther and ever farther until, 

 at the extreme limit of our telescopes, we reach the last outposts of the 

 observable region of the universe. With the 100-inch reflector, the 

 great spiral in Andromeda could still be recognized as a nebula if it 

 were so remote that its light had to travel for a thousand million years 

 to make the journey. 



