1888.] President's Address. xli 



Hemisphere was of a very meagre description ; but now, thanks to 

 the labours of these two astronomers and their assistants, we can 

 safely say that the southern half of the sky is better surveyed than 

 the northern ; and it will remain so till the publication of the 

 catalogue of stars to the 9th magnitude now being made under the 

 direction of the Ast. Gesellschaft and by the co-operation of fourteen 

 observatories in Europe and America. In 1860 Dr. C. H. F. Peters 

 commenced the enormous task of forming maps giving the positions 

 of all stars down to the 14th magnitude situated within 30'' on each 

 side of the ecliptic. Part of the work was finished in 1882 and will 

 prove invaluable in the search for minor planets ; but the photographic 

 survey of the sky, which was resolved on last year and which will 

 shortly be commenced, will replace all similar work in the future. 

 ,'Such a survey, including all stars to the 9th magnitude, is now 

 -approaching completion at the Cape Observatory, and the work of 

 measuring the photographs and cataloguing the stars is being done 

 by Prof. Kapteyn of Groningen. Variations in the brightness of 

 stars have always attracted much attention and there are many striking 

 instances of such changes, e.g.^ the wonderful i] Argus which 50 years 

 ago blazed out till it was the brightest star in the southern skies, 

 while now it is below the limit of visibility to the unaided eye. 

 Extensive catalogues have recently been made giving accurate 

 determinations of the magnitudes of the stars, by Gould for the 

 Southern Hemisphere and by Pritchard and Pickering for the Northern. 

 Gould's estimates were made with the naked eye or by the help of 

 an opera-glass ; Pritchard's by means of the *^' wedge photometer," 

 a wedge of neutral -tinted glass mado to slide in front of the eyepiece 

 of the telescope till the point of extinction of the star's light is 

 reached ; Pickering's by a polarising apparatus used in such a way 

 as to reduce the light of the stars to equality. Successful photographs 

 of stellar spectra were obtained by Huggins in 1879 and by Gornu 

 in 1886, and this work is now being carried on at the Harvard Obser- 

 vatory on an extensive scale. A large prism placed in front of the 

 object-glass gives at once the spectra of all the stars in the field of 

 view of the telescope ; and in January 1886, with an exposure of 

 34 minutes a photograph was obtained shewing the spectra of 40 

 stars in the Pleiades, which nearly all belonged to the same type. 



Determinations of the velocities of stars in the line of light have 

 been regularly kept up at Greenwich and by Vogel, but the excessive 

 iiifficulty of the observation has prevented the results hitherto obtained 



