PRKSIJJKXriAL A])J>KKSS SKCTIOX A. 39 



pronounced as to .suo<)'est that we are not dealino- with a 

 distribntion of steUar motions in one system such that the 

 veh^city surface is aji ellipsoid with one relatively long- axis, 

 but that we are actually dealing with two definite streams of 

 stars. The suggestion is made here tentatively, for further 

 work has to be done, that the results obtained support the 

 view that in the near vicinity of the Sun we have two star 

 clusters intermingling and passing through one another. 

 Further, the comparison between the blink-microscope 

 motions for bright stars, bright enough to figure in meridian 

 catalogues, with the catalogue results for the motion shows 

 the hypothesis that the background of stars is at a relatively 

 great distance to be fairly correct. 



Much new work has been done recently in the deter- 

 mination of stellar parallaxes by indirect methods. These 

 indirect methods all depend upon the simple principle that 

 if you know the absolute magnitude of a star you can 

 determine its distance from us from an observation of its 

 apparent magnitude. If the lights of a town are seen from 

 a distant elevated point, the more distant street lamps 

 appear to be much fainter than the nearer ones, and, if it 

 is known that the same type of street lamp is in use 

 throughout the town, the distance of the far relatively to the 

 near ones can be deduced from the apparent brightness of 

 the two. If the distance of the near lamps is known, the 

 actual distance of the more distant lamps can be calcidated. 

 Hence, if a number of stars of the same absolute magnitude 

 can be recognised in the sky, the more distant ones will have 

 a relatively greater (fainter) magnitude than the near ones. 

 If the distance of the near ones has been determined by the 

 direct method of determining stellar parallaxes, then the 

 actual distances of the fainter and more distant ones can be 

 deduced from the relation between the apparent magnitudes. 



There are various ways in which the absolute magnitude 

 of a star can be estimated. Adams and Kohlschutter, in 

 1917, detected differences in the relative intensity of certain 

 lines in stellar spectra depending upon the absolute magnitude 

 of the star. Consequently, by comparing spectra of stars 

 of the same type they can deduce the absolute magnitude of 

 the star, and then from its apparent magnitude its distance. 

 This is a wonderful extension of the work of the spectroscope, 

 which was used primarily to find the chemical constituticni 

 of a star, then its velocity, and now its distance. 



There is a certain class of stars designated the helium 

 stars. There are many reasons for considering that such 

 stars are at the zenith of stellar evolution and are the hottest 

 stars. They are the most massive of the stars, and, further, 

 they all have approximately the same mass. Hence, if a 

 star is found by the spectroscope to be a helium star, its 

 distance can at once be deduced from its apparent brightness. 

 Thus the onlv limit to determining the distance of a helium 

 star is that ' imposed by the spectroscope — there must be 

 enough light to show the character of its spectrum. 



