144 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1925 



that there is a slight dependence of the surface brightness upon the 

 api^arent diameters in the sense that the surface brightness slightly 

 decreases as the nebula is more distant. For the explanation of this 

 it is natural to have recourse to a general absorption in the space 

 traversed. It may be due to the weakening of the light in passing 

 through an extremely tenuous medium. The selective absorption 

 necessarily bound with the phenomenon has not been demonstrated. 

 If we evaluate the amount of absorption, it is approximately such 

 that when the light passes through a .space 3,000 light-years thick 

 the light is weakened 0,00001 magnitude, an amount too small to 

 be effective within our own galax}^ 



Bizarre as it may seem, the limit of the system of spiral nebulai 

 is apparently accessible to our researches, which now extend about to 

 the confines of our stellar galaxy. We may consider that we are 

 approaching this limit if the number of new nebulai discovered by 

 more and more powerful telescopes grows smaller and smaller, or at 

 any rate does not augment as the optical powers of our instruments 

 increase. We may add that the cumulation of nebular matter toward 

 the north pole of the millcy way diminishes as we push farther out 

 into space toward the fainter and fainter nebulae. Statistics relative 

 to the apparent distribution of the nebula3 lead to this result. Com- 

 parison with the relative distribution of the stars, determined by 

 similar means, leads to the conclusion that we should not look to the 

 spiral nebulae as the birthplace of our stars. 



There is a fact which fits with difficulty into the theory which 

 considers the spiral nebulae as distant milky ways. That is the 

 complete independence of their surface brightness upon their ori- 

 entation. For, seen from the depths of space, our milky way would 

 appear five times as luminous seen edgewise as broadside on. Fur- 

 ther, the surface brightness of the milky way as a whole would be 

 small compared with that of the spiral nebulae. The milky way sys- 

 tem is therefore exceptional, and its brightness places it at the lower 

 rank of its class and as an extreme in any scheme of the evolution 

 of spiral nebula3. It is not in our power to say whether to place it 

 at the beginning or end of the evolution. 



We see that what has been stated relative to several problems 

 which are connected with the spiral nebulae, or in a more general way 

 with cosmical nebulae, leads us, literally and metaphorically, to the 

 limits of our knowledge and the confines of space, where reality 

 and fiction, where the physical world and the shadows of our 

 thought confound. Meanwhile neither this world nor science stop 

 there. There is the possibility of finding a response to ques- 



