1912.] DEPTH OF THE MILKY WAY. 7 



us, does cease after a certain faintness and distance has been 

 reached; but is equally certain that other clusters or clouds of 

 helium stars recur at greater distances, among the millions of small 

 white stars constituting the Milky Way. For as Herschel long ago 

 noticed the Galaxy is everywhere observed to traverse the circuit 

 of the heavens in a clustering stream; and our view of it from the 

 region of the sun is not essentially different from the view that 

 could be obtained from other points in this starry stratum. Add 

 to this consideration the fact of the well-known whiteness of the 

 small stars in the Milky Way, and we are authorized to conclude 

 that an indefinite number of clusters or groups of helium stars will 

 be found in the Milky Way, and thus such stars will certainly exist 

 at the greatest depths to which our giant telescopes can penetrate. 



We must therefore be on our guard against the superficial view, 

 that because the helium stars near the sun fade away as the sixth 

 magnitude is approached, other groups of stars of this type do not 

 occur at greater distances. The typical whiteness of the millions 

 of small stars which make up the Milky Way, and the clustering 

 character of that magnificent collection of stars, alike forbid any 

 such inference. 



Herschel had the correct view of the constitution of the Galaxy 

 a century ago. Unfortunately his works have been very inaccessible, 

 and are so little used that many erroneous conceptions have been 

 given currency by more superficial investigators. It is impossible 

 to commend too highly the movement now on foot in England to 

 reissue the collected works of Sir William Herschel. In all that 

 pertains to the sidereal universe as a whole he is easily the greatest 

 of all modern astronomers, and will always remain unrivaled. 



§ 4. Explanation of the Methods Employed by Campbell for 



Finding the Average Distance of the Group of 



Naked Eye Helium Stars. 



This is essentially a combination of the line of sight motion as 

 found at Lick Observatory, with the proper motions resulting from 

 observations with the meridian circle, by many observers, as worked 



