MOUNT WILSON SOLAR OBSERVATORY. 201 



revealed the prevalence in these objects of faint temporary stars, 

 whose tendency to form in the branches of spiral nebulae strongly sug- 

 gests an important analogy with the well-known fact that bright 

 temporary stars invariably blaze out in or near the Milky Way. The 

 classic problem of "island universes" is again under vigorous discus- 

 sion, and much attention will be paid here to its further investigation. 



The distances of the stars, so fundamental a factor in all astro- 

 nomical studies, whether structural or physical, are now determined by 

 two methods. The first involves the simple trigonometrical principle 

 of measuring the displacement of a star's position, as observed from 

 opposite extremities of the earth's orbit. From this accurately known 

 base-Hne their distances are readily calculated, precisely as in terres- 

 trial triangulations. The high precision of differential measurements 

 obtainable with such an instrument as our 60-inch reflector would seem 

 to make this method very reliable. Insidious systematic errors, how- 

 ever, may enter to vitiate the conclusions. It is therefore satisfactory 

 to find, as the result of careful study, that these are apparently very 

 small. 



The use of the second method, recently devised here and described 

 in the last annual report, has led to extensive and highly significant 

 developments. By this method every good photograph of a star's 

 spectrum yields a measure of its distance, given by a simple de- 

 termination of the relative intensities of certain lines. The limita- 

 tion of application to the more advanced types of spectra has been 

 partially removed by the inclusion of new groups of lines, which serve 

 as criteria in the earlier types. Moreover, any doubts that may have 

 existed as to the accuracy of the method have been dispelled by a com- 

 parison of the spectroscopic and trigonometric parallaxes of 360 stars, 

 which shows an exceedingly satisfactory agreement. The rapidity 

 and simphcity of the method will therefore at once lead to extensive 

 applications. 



The first of these, involving an investigation of the relationship 

 between the absolute magnitudes and motions of some 1,300 stars, 

 elearly indicates an increase of velocity with decrease in absolute 

 magnitude, independent of distance from the sun. The conclusion 

 that the intrinsically fainter stars move more rapidly than the brighter 

 ones must prove of exceptional significance in the theory of stellar 

 evolution, -\nother interesting conclusion, in harmony with previous 

 results for stars of earlier types, is the marked condensation of the 

 later-type stars of high luminosity in the plane of the Milky Way. 



The extensive task of determining the photographic magnitudes of 

 stars in 115 of the Selected Areas of Kapteyn has been essentially 

 completed, and work on the photovisual magnitudes is also advancing. 

 The former results have afforded means for a study of the increase in 

 the number of the stars toward the plane of the Milky Way. From 



