( 1^8 ) 



group W and X Sagittarii which also yield small values; as has 

 been remarked, for the southern stars no distinction is made between 

 the a and the c stars ^). 



We may expect that within a few years our knowledge of the 

 orbits of the spectroscopic double stars will have augmented consi- 

 derably. Then it will be possible to derive conclusions like those 

 found here from much more abundant material, and also to arrive 

 at some certainty about the mean mass of the K stars. With regard 

 to the latter our results show at any rate that in investigations on 

 grouping of stars and stellar motions it will be necessary not to 

 consider the 2°^ type as one whole, but always to consider the 

 F and G stars apart from the redder K stars. 



1) In this connection may be mentioned that in 1891 the author thought he 

 detected a variability of u Ursa e minoris with a period of a little less than 4 days. 

 The small amplitude and the great influence of biased opinions on estimations of 

 brightness after Argelander's method in cases of short periods of almost a full 

 number of days, made it impossible to obtain certainty in either a positive or a 

 negative sense. Campbell's discovery that it is a spectroscopic binary system with 

 a period of 3'^ 23^ l^™ makes me think that it has not been wholly an illusion. 



ERRATA. 



In the Proceedings of the Meeting of June, 1905, p. 81 : 

 line 7 from top, read: "cooled by conduction of heat", 



16 „ „ for: ''Exh' PI. IV" read: ''Exit' PI. VI". 

 In Plate V belonging to Communication W. 83 from the physical 

 laboratory at Leiden, Proceedings of the Meeting of February 1903, 

 p. 502, the vacuum glass B\ has been drawn 18 cm. too long. 



(August 21, 1906). 



