May 25, 1917] 



SCIENCE 



533 



Fig. 21. The Spiral Nebula, Messier 33, photographed by Keeler with the 

 Ci-ossley Eeflector of the "Lick Oljservatory. 



proaching re.solutioii in certain iDarts of one 

 great spiral, Messier 33, in the constellation 

 of Triangulnm (Pig. 21). That one is 

 probably relativelj^ near ns, but of that we 

 are not sure. Some astronomers who have 

 photographed the Magellanic clouds (see 

 Figs. 22 and 23), in the far southern sky, 

 say that these remarkable objects are spiral 

 in structure. If so, they are spirals easily 

 resolved into stars. Nevertheless, these 

 clouds are enormously distant. Hertz- 

 sprung has estimated, from certain consid- 

 erations, that the distance of the Lesser 

 Cloud is of the order of 30,000 light-years. 

 Wilson has measured the spectroscopic ve- 

 locities of approach and recession of twelve 



nebulaa in the Greater Cloud, and has found 

 that they are receding with speeds lying be- 

 tween 150 and 200 miles per second. The 

 velocities of the twelve objects differ from 

 each other as many miles per second as we 

 should expect to find for twelve bright-line 

 nebulffi selected at random in our galactic 

 system, but the average for the twelve in 

 the Cloud shows an abnormally high rate 

 of recession. All of the known bright-line 

 nebulfe in that great region of the sky are 

 in the two Clouds. None is known to exist 

 outside of the Clouds until we have gone 

 far from them in the direction of the Milky 

 Way (see Fig. 11). There are still other 

 reasons for the unquestioned conviction 



