It appeared on reflection that the surest way to 

 accomplish our own special object was to coop- 

 erate in the closest possible w r ay with Kapteyn, 

 even to the extent of devoting a large share of the 

 working-time of our instruments to the study of 

 his problems. The physical development of stars 

 may depend upon just such association in systems 

 as the discovery of star-streams has disclosed, and 

 many questions might long escape answer if at- 

 tacked only from a single viewpoint. In short, 

 close cooperation should prove mutually advan- 

 tageous, contributing in material measure toward 

 the solution of what, after all, is but a single great 

 problem. How manifestly the annual visits of 

 Kapteyn to Mount Wilson have aided progress 

 toward our original goal will appear in the sequel. 



INSTRUMENTAL POSSIBILITIES. 



And now a word as to instrumental means. 

 Here we may advance in two ways: (i) as w r e have 

 seen, by the use of methods and the adaptation of 

 principles, borrowed in the main from the phys- 

 icist, and (2) by increase in telescopic power, 

 derived from greater optical aperture and greater 

 perfection of optical and mechanical construction. 

 Mere size is of no moment, unless supported by 

 corresponding precision of parts. Lord Rosse's 

 6-foot reflector, built before the development of 

 modern machine tools, was less efficient than a 

 12-inch telescope of the present day. It is true 

 that photography is mainly responsible for the 



