STRTJCTUEB OF THE GALAXY PLASKETT 209 



Nevertheless, the evidence in favor of the rotation seems over- 

 whelming and it will be useful to summarize it briefly: (a) The 

 spiral nebulae, to which strong evidence points as being replicas of 

 the galaxy on a smaller scale, are by their very appearance obviously 

 in rotation, while rapid rotation has been spectroscopically meas- 

 ured in several of them, (h) The extremely flattened shape of the 

 galaxy, with the thickness only about one-twentieth the diameter, 

 is in itself almost indisputable evidence of rapid rotation in its 

 own plane ; otherwise it would assume a more nearly spherical form 

 like a globular cluster, (c) The very presence of the cosmic diffuse 

 matter, obeying the differential rotational swing exactly as the stars, 

 is in itself a proof of galactic rotation. Only rotation could have 

 kept this gas distended throughout the system and have prevented 

 it from long ago collapsing into a dense nebula at the center. 

 (d) The satisfactory manner in which the galactic rotation has 

 accounted for the previously unexplained systematic effect of star- 

 streaming and the asymmetry in the high-velocity stars also counts 

 in its favor, (e) The observed residual velocities of the most dis- 

 tant stars and of the cosmic cloud agree so closely with those that 

 would be produced by a rotation of the galaxy as to make any other 

 explanation highly improbable. (/) And finally the dimensions of 

 the system and the direction to the center as determined dynamically 

 from the anaWsis of the velocities agree almost exactly with those 

 determined by the direct geometrical measures of distance and posi- 

 tion of the center. 



I have been attempting this evening, with I hope some success, to 

 carry you with me in imagination away from our terrestrial limi- 

 tations, not to the bounds of space which are as yet glimpsed un- 

 certainly, if at all, but only sufficiently far to give a comprehensive 

 view of the galaxy. "VVe should now be able to see it, not as an 

 unorganized aggregation of stars and star clouds, still less as a 

 haphazard collection of widely separated nebulae, but as a great 

 dynamical unit of definite discoidal form rotating in its own plane 

 in a majestic and beautifully ordered way. The galaxy is a wonder- 

 ful example of the universal reign of law in the physical world and 

 of the beneficent wisdom and power of the Supreme Ruler of the 

 universe. 



NOTES ADDED HY AUTHOR, APRIL, 19 34 



Developments in astronomy in the 2 years since this lecture was siven have 

 considerably modified some of the dimensions there given and the conclusions 

 reached, and it seemed desirable to incorporate the changes in an appendix 

 rather than modify the text. 



The principal development has been the general acceptance by astronomers 

 of the presence of some kind of an absorbing medium in the galaxy, principally 

 in the neighborhood of the central plane, which both dims and reddens the light 



