May 25, 1917] 



SCIENCE 



535 



Seed 27, 20 min. Iso, 2 hours. 



Fig. 24. The Spiral Nebula, Messier 99, photographed -with- 

 out and with absorbing color screen, by Seaj'ss with the 60-inch 

 reflecting telescope of the Mount Wilson Solar Observatory. 



scarce. No one has given a satisfactory 

 answer to tliis unless it be correct to say 

 that these great globular clusters of stars, 

 though extremely distant, are yet near at 

 hand in comparison with the distances of 

 the spirals. Ritchey's photograph of the 

 great cluster in Hercules (Fig. 25) records 

 fully 30,000 stars, and these are undoubt- 

 edly only the more brilliant members of the 

 cluster. Still longer exposures might re- 

 cord 100,000 stars and yet leave unrecorded 

 the fainter members of the cluster in vastly 

 greater numbers. Shapley estimates the 

 distance of this cluster, from several lines 

 of investigation, as of the order of 100,000 

 light-years. Other great clusters have not 

 been so thoroughly studied, but the avail- 

 able evidence concerning them is in har- 

 mony with the hypothesis of their great dis- 

 tances. Our own universe may be enor- 

 mously more extensive than we see it be- 

 cause the outer stretches of it may be hid- 

 den by obstructing materials. If the ob- 

 structing materials consist chiefly of dis- 

 crete particles whose diameters are large in 

 comparison with the wave-lengths of light, 

 we should expect the obstruction to be such 

 as to reduce the brightness of distant ob- 

 .jects without changing seriously the quality 

 of their light. 



The most elaborate structure yet pro- 

 posed to explain the origin and develop- 

 ment of the solar system is the planetesimal 

 hypothesis by Chamberlin and Moulton. 

 They postulate that the materials now com- 

 posing the sun, planets and satellites, at one 

 time existed as a spiral nebula, or as a 

 great spiral swarm of discrete jDarticles, 

 each particle in elliptic motion about a cen- 

 tral nucleus. The authors go further back 

 and endeavor to account for the origin of 

 spiral nebulse, but it should be said that this 

 phase of the subject is not vital to their 

 hypothesis. It will happen once in a while, 

 they say, that two massive stars will ap- 

 proach and pass each other closely. These 

 stars will raise great tidal waves upon each 

 other by their mutual attractions, and there 

 will be outbursts of matter from each body, 

 not only on that side of each which faces the 

 other body, but on the opposite side of each, 

 for somewhat the same reason that tidal 

 waves in our oceans are raised on the side 

 opposite the moon as well as on the side 

 toward the moon. They assume that a great 

 star traveling in what is now the principal 

 plane of our solar system passed close to 

 our sun when it was in an earlier stage of 

 its existence ; that a resulting disruption of 



