56 NATIONAL ACADEMY OF SCIENCES 



ADDRESS OF DOCTOR J. C. KAPTEYN 



ON 



THE STRUCTURE OF THE UNIVERSE 



DR. KAPTEYN: Ladies and Gentlemen: I have been asked 

 to address you on the structure of the universe. The title is 

 ambitious, and I fear that what I have to say on the subject will 

 be sadly in disproportion with what some of you will be led to 

 expect by this title. 



It will, however, I hope, give you a glimpse of what astron- 

 omers nowadays are attempting to do, in order to penetrate 

 somewhat into the mystery of the starry sky. 



The problem, as I take it, is a double one. We have, first, the 

 structure of the universe as it is at the present moment; and this 

 problem is, in the main, no other than finding the star distances, 

 because the star directions we can readily ascertain. 



We have, second, the problem of the history and evolution of 

 the system. 



As the time at my disposal is so short, I must confine myself 

 to one of the two, and undoubtedly the first is at the present 

 moment the more promising, owing to the recent discovery 

 of star-streaming, and due furthermore to the history of the 

 system during the past ages, ages to be counted by millions, and 

 probably hundreds of millions of years, which is and perhaps 

 will remain enshrouded in great mystery. Still I think that the 

 second problem of the evolution of the system may, perhaps, be 

 the more suitable subject for the present lecture. 



You will all, of course, understand, without my saying any- 

 thing to the purpose, that what we have to expect can not well 

 be anything else than a few more or less probable inferences 

 about the course of events that have made our system what it is. 



Some additional considerations might easily have been added, 

 but as I have had to give up the idea of giving a general review 



