532 



SCIENCE 



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



of the stellar sj'stem, with the solar sj^stem 

 somewhere near its center. This is an as- 

 sumed distribution which leads straight into 

 difficulties and to some degree of absurdity. 

 We should have to saj^ that the spirals live 

 close to the right of us and close to the left 

 of us, but that they avoid getting between 

 us and the Milky Waj^ structure. 



Slipher has measured the motions of ap- 

 proach and recession of more than a score 

 of prominent spirals, and he finds they are 

 moving with speeds surprisingly high, run- 

 ning up to 600 or 700 miles per second, with 

 an average of roughly 250 miles per sec- 

 ond. Now, if they are moving at random, 

 which is the probable approximate truth, 

 their average speed at right angles to the 

 line of sight must be still higher, and their 

 average speed in space would be of the 

 order of 500 miles per second. There is no 

 class of objects loiown to exist within our 

 stellar system which have velocities at all 

 approaching that scale. 



Curtis has conducted an extensive in- 

 vestigation to determine whether and how 

 much the principal spiral nebulae have 

 moved on the surface of the sky in the last 

 fifteen years, on the basis of photographs 

 taken at the beginning and at the end of 

 that period. Lampland and van Maanen 

 have similarly sought for evidences of 

 change of position of three or four spirals. 

 The indications of motion in all these cases 

 are so slight as to leave us uncertain 

 whether the motions of the brighter and 

 larger spirals across the face of the sky in 

 fifteen years have been sufficient for de- 

 tection. Harmonizing this fact of their 

 vanishingly small angular motions with 

 Slipher 's high speeds in miles per second, 

 we conclude that they must be enormously 

 distant, and therefore enormously large 

 bodies. 



"With the spectrograph, Slipher has been 

 able to measure the rotational speeds of two 

 or three spirals, including the great spiral 



in Andromeda, and Pease has repeated and 

 extended the experiment on one of them. 

 The rotational speeds are also extremely 

 rapid, as indeed one should expect from the 

 tremendous inequalities in what we venture 

 to call their equatorial and polar diameters. 

 Assuming that Newton 's law of gravitation 

 controls their rotations, the probable masses 

 are stupendous. Some of thera seem to con- 

 tain enough material to make tens of thou- 

 sands, probably hundreds of thousands, and 

 possibly millions, of stars comparable in 

 mass with our own sun. 



The spectra of only a few of the brighter 

 spirals have thus far been investigated in 

 any degree of adequacj^, but they have the 

 characteristics which we should expect to 

 find if the spirals consist chiefly of multi- 

 tudes of stars. I say chiefly, because in the 

 spectra of some of them we find the bright- 

 line spectrum of gaseous nebulfe superposed 

 upon what we may call the stellar tj'pe of 

 spectrum. 



If we carried our spectrograph so far out 

 into space that, looking back, our stellar 

 system would be reduced down to the ap- 

 parent size of the well-known spiral nebulae, 

 and we turned the instrument upon our 

 condensed system, we should expect to see a 

 spectrum very like the continuous and ab- 

 sorption-line spectra yielded by the spirals ; 

 and certain structures in our sj'stem, such 

 as the region containing the Orion nebula, 

 might well yield the bright nebular lines 

 found in some parts of certain spirals. Cur- 

 tis has recently examined the spectra of 

 fift.y of the brighter small nebulffi lying 

 within 25° of the Galaxy which had not 

 been subjected earlier to spectroscopic test. 

 None of them showed bright lines; all ap- 

 peared to be of the stellar or cluster type of 

 spectrum. An extension of the survey to 

 the pole of the Galaxj^ would probably have 

 had similar results. 



We are not able to resolve the spirals 

 into stars, except as we seem to be ap- 



