300 Professor Dr. J. C. KapUyn [May 22, 



WEEKLY EVENING MEETING, 



Friday, May 22, 1908. 



Sir William Crookes, D.Sc. F.R.S., Honorary Secretary and 

 Vice-President, in the Chair. 



Professor Dr. J. C. Kapteyn, 



Director of the Astronomical Laboratory , and Professor of Astronomy 

 in the University of Groningen. 



Recent Researches in the Structure of the Universe. 



Introduction. 



I CONSIDER it an uncommon privilege to lecture on the Structure of 

 the Universe in the country of the Herschels. 



Even now their celebrated gauges are unrivalled, 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 

 directing the telescope successively to different pomts 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 Milky-way, that belt which 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-way, we come 

 to the poorest parts of the sky. 



Variation with Galactic Latitude. 



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

 direct our telescope first towards the part of the Milky-way near 

 Sirius, and if from thence we gradually work up towards the North 

 Pole of the Milky-way in the constellation called the Hair of Berenice, 

 we shall clearly perceive this gradual and regular change in the 

 number of stars. Now if we repeat the same process, beginning from 

 some other point of the Milky-way, say in Cassiopeia, or the Southern 

 Cross, we shall find that, not only is there a similar gradual 

 change, but we shall approximately go through the same changes. 



