534 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



that these nebulaj are actualh' a part of the 

 structure of the Clouds. It is difficult to 

 avoid the conclusiou that the observed speed 

 of recession of the twelve nebulas within the 



Pig. 22. The Greater Magellauie Cloud, photo- 

 graphed by Bailey, at the Arequipa, Peru, station 

 of the Harvard College Observatory. 



Greater Cloud, averaging 175 miles per sec- 

 ond, is the approximate speed of reces- 

 sion of the Greater Cloud. Motions of this 

 magnitude have not been observed for any 

 objects known to be within our stellar sys- 

 tem, except in the case of three or four in- 

 dividual small stars. We can not seriously 

 doubt that the Magellanic Clouds are dis- 

 tinct from and independent of our great 

 stellar system ; and, if they are of spiral 

 structure, they are spirals relatively near 

 to us, as the distances of spirals go. 



Scares has recently discovered an inter- 

 esting peculiarity of spiral nebula, at least 

 of the four or five thus far studied, by 

 means of photographs of spirals, with ex- 

 posures first on ordinary plates sensitive to 

 the blue-violet rays, and secondly on iso- 

 ehromatic plates and through screens which 

 transmit only the yellow-green rays. The 

 results, illustrated in Pig. 24, show that the 

 light from the outer arms of the spirals is 

 richer in the blue and violet rays than is the 



light from the central nuclei. The signifi- 

 cance of these facts is not yet clear. 



We can not say that the problem of in- 

 terpreting- the spiral nebulaj has been 

 . solved. In fact, it is a fair statement that 

 our positive knowledge as to the conditions 

 existing within them is regretfully meager. 

 We are not certain Jiow far away they are; 

 ive are not certain luhat they are. How- 

 ever, the hypothesis that they are enor- 

 mously distant bodies, that they are inde- 

 pendent systems in different degrees of de- 

 velopment, is the one which seems to be in 

 best harmony with known facts. It should 

 be said that this hypothesis is a very old 

 one. Swedenborg speculated upon the idea 

 that our stellar system is but one of a great 

 number of systems. The serious considera- 

 tion of the hypothesis, upon the basis of ob- 

 served facts, may be said to date from the 

 two Herschels. Following Keeler's epoch- 

 making observations of nebula in 1898- 

 1900, many astronomers have studied the 

 subject. 



We naturally ask why it is that certain 

 globular star clusters are visible not far to 

 one side or the other of the Milky Way, 

 whereas the spirals there are faint and 



Pig. 23. The Lesser Magellanic Cloud, photo- 

 graphed by Bailey, at the Arequipa, Peru, station 

 of the Harvard College Observatory. 



