graph their spectra side by side on a single plate. 

 Since blue and violet light is weakened by scatter- 

 ing more than red, we should expect the violet part 

 of the spectrum of the more distant star to be 

 fainter than that of the nearer star. In general, 

 this proved to be the case. 



Many spectra of near and distant stars were 

 then selected from our collection of photographs 

 and divided into groups, each representing a dif- 

 ferent stage of evolution. That is to say, in 

 terms of our present view of stellar ages, young, 



FIG. 50. Weakness in Violet of the Spectra of Distant Stars. 

 Compare middle spectrum of each group with those above and 

 below. 



middle-aged, and old stars were placed in separate 

 groups. It was found that in all of the groups, 

 with one possible exception, the more distant stars 

 are redder than the nearer ones, but that the dif- 

 ference in color also depends in some way upon the 

 age of the star. 



Investigations are now in progress to explain 

 this, but the advantage of dealing with Kapteyn's 

 problem of distance from the astrophysical stand- 

 point is already evident. On the one hand, an 

 influence of physical development has been found 



