EECENT RESEARCHES IN THE STRUCTURE OF THE 



UNIVERSE." 



By Prof. Dr. J. C. Kapteyn, 



Director of tlte Astronomical Laboratory, and Professor of Astronomy in the 



University of Groningen. 



nSTTRODITCTTON. 



I consider it an nncommon privilege to lecture on the structure of 

 the universe in the country of the Herschels. 



Even now their celebrated gauges are unrivaled, and they still 

 form one of the important elements on which any theory of the 

 stellar system must be based. 



It is well known that the plan of these gauges consisted in direct- 

 ing the telescope successively to different points all over the sky, and 

 simply counting the number of stars visible in the field. 



REGULARITY IN THE ASPECT OF THE SKY. 



There is one fact clearly brought out by these gauges to which I 

 must call your attention. It is that in the outward appearance of 

 our nightly sky, as seen with the telescope, there is a great regularity. 

 In the INIilky Way, that belt Avhich we see with the naked eye 

 encircling the whole of the firmament nearly along a great circle, the 

 number of stars, as seen in Herschel's 20-foot reflector, is enormous. 

 On both sides this apparent crowding of the stars diminishes very 

 gradually and regularly, till near the poles of the Milky AVay we come 

 to the poorest parts of the sk}^ 



VARIATION WITH GALACTIC LATITUDE. 



Let us look at this phenomenon somewhat more closely. If we 

 direct our telescope first toward the part of the Milky Way near 

 Sirius, and if from there we gradually work up toward the North 

 Pole of the Milky Way in the constellation called the " Hair of 



" Lecture before the Royal Institution of Great Britain, London, Friday, 

 May 22, 1908. Reprinted by permission. 



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