SLIPHER— NEBULA. 409 



when more are observed there, still others will be found to have 

 approaching motion. It is unfortunate that the twenty-five ob- 

 served objects are not more uniformly distributed over the sky as 

 then the case could be better dealt with. It calls to mind the 

 radial velocities of the stars which, in the sky about Orion, are 

 receding and in the opposite part of the sky are approaching. This 

 arrangement of the star velocities is due to the motion of the solar 

 system relative to the stars. Professor Campbell at the Lick Ob- 

 servatory has accumulated a vast store of star velocities and has 

 determined the motion of our sun with reference to those stars. 



We may in like manner determine our motion relative to the 

 spiral nebulas, when sufficient material becomes available. A pre- 

 liminary solution of the material at present available indicates that 

 we are moving in the direction of right-ascension 22 hours and 

 declination — 22° with a velocity of about 700 km. While the 

 number of nebula is small and their distribution poor this result 

 may still be considered as indicating that we have some such 

 drift through space. For us to have such motion and the stars 

 not show it means that our whole stellar system moves and 

 carries us with it. It has for a long time been suggested that the 

 spiral nebulas are stellar systems seen at great distances. This is the 

 so-called "island universe" theory, which regards our stellar sys- 

 tem and the Milky Way as a great spiral nebula which we see from 

 within. This theory, it seems to me, gains favor in the present 

 observations. 



It *is beyond the scope of this paper to discuss the different 

 theories of the spiral nebulae in the face of these and other observed 

 facts. However, it seems that, if our solar system evolved from a 

 nebula as we have long believed, that nebula was probably not one 

 of the class of spirals here dealt with. 



Our lamented Dr. Lowell was deeply interested in this investi- 

 gation as he was in all matters touching upon the evolution of our 

 solar system and I am indebted to him for his constant encourage- 

 ment. 



Lowell Observatory, 

 April, 1917. 



