THE ORIGIN OF THE EARTH. 43 



and yet they appear to intercept but little light, and give no signs of 

 great attractive power. 



The dominant form of these nebulae is the spiral, as determined by 

 the late Professor Keeler, 1 and this form particularly characterizes the 

 smaller nebulae recently brought to knowledge by improved instru- 

 ments and manipulative skill. These newly discovered nebulae are 

 estimated to number at least ten times the whole number previously 

 known. From the superior number of spiral nebulae it is a safe inference 

 that their peculiar forms represent some prevalent process in celestial 

 dynamics. That is, in itself, a reason why research should turn to 

 them by preference for the origin of the present solar system. 



Nothing is yet positively known of the motions of the parts of these 

 spirals, for time enough has not yet elapsed since they were first sharply 

 photographed to permit the requisite comparisons. Inferences from 

 their remarkable forms are the only present resource. The accom- 

 panying photographic illustrations of some of the larger and more 

 distinct forms will permit independent judgment as to the validity of 

 these inferences (Figs. 12 to 21). To us their peculiar forms seem 

 to imply that the spirals sprang from a combined outward and rotatory 

 movement. The outward movement may have been already checked 

 by the gravity of the central mass, and the rotatory motion be domi- 

 nant at present, but their forms seem still to bear the impress of an 

 outward movement. If the outward movement has ceased, or when it 



1 The profoundly lamented death of Professor Keeler, just as he was beginning 

 to reap the rich fruits of his skill and patience in nebular investigations, gives 

 historical value to his latest statement of results, published about two months 

 before his death. 



"1. Many thousands of unrecorded nebulae exist in the sky. A conservative 

 estimate places the number within reach of the Crossley reflector at about 120,000. 

 The number of nebulas in our catalogues is but a small fraction of this. 



"2. These nebulae exhibit all gradations of apparent size from the great nebula in 

 Andromeda down to an object which is hardly distinguishable from a faint star-disk. 



"3. Most of these nebulae have a spiral structure. . . . 



"While I must leave to others an estimate of the importance of these conclusions, 

 it seems to me that they have a very direct bearing on many, if not all, questions 

 concerning the cosmogony. If, for example, the spiral is the form normally assumed 

 by a contracting nebulous mass, the idea at once suggests itself that the solar sys- 

 tem has been evolved from a spiral nebula, while the photographs show that the spiral 

 nebula is not, as a rule, characterized by the simplicity attributed to the contract- 

 ing mass in the nebular hypothesis. This is a question which has already been taken 

 up by Professor Chamberlin and Mr. Moulton of the University of Chicago." — As- 

 trophys. Jour., June 1900, pp. 347,348. 



