Annual Address to the Members. Iv 



Annals of the Cape Observatory, and contains the approximate places 

 and magnitudes of 454,875 stars, under the title of the Cape Photo- 

 graphic Durchmusterung. 



But the experiments which led up to the Cape Durchmusterung 

 had a still more important result. They were the means of setting 

 on foot the experiments of the brothers Henry in Paris on the con- 

 struction of astronomical photographic object glasses, — experiments 

 which met with such success as to lead me, on June 4, 1886, to 

 propose an International Astro-photographic Congress, with a view 

 of considering how this now perfected method of observation could 

 be most efficiently applied to the complete cartography of the sky. 

 The proposal was favourably received by Astronomers generally, 

 and Admiral Mouchez, then Director of the Paris Observatory, threw 

 himself with so much ardour into the promotion and encourage- 

 ment of the scheme, that in 1887 a general congress of fifty-seven 

 Astronomers from all parts of the world met at Paris under his 

 presidency to consider what international action should be taken to 

 promote its full development. It is greatly owing to his earnestness, 

 his tact and large-hearted sympathy that the meeting ended with 

 complete success, and with an unanimous resolution to pursue a 

 definite and united programme for cataloguing all the stars to the 

 11th order of magnitude and for making chart plates of the whole 

 sky, including stars of the 14th magnitude. Further meetings of 

 the Permanent Committee appointed for the execution of the work 

 were held in 1889, 1891, 1893, 1896, and 1900. The next will pro- 

 bably be held in 1904. The sky was divided into zones amongst 

 eighteen different observatories, most of which have made good 

 progress with their share of the work. Three South American 

 observatories, which for various reasons had made no progress, were, 

 in 1900, replaced by those at Cordoba, Monte Video, and Perth 

 (Western Australia). The Cape work makes satisfactory progress, 

 and many discoveries crop up by the wayside as we proceed. 



Amongst the most interesting of these are the variable stars. 



South Africa has been the chief seat of study of variable stars in 

 the Southern Hemisphere, and Dr. Eoberts of Lovedale its most 

 accomplished student. He began observing variable stars in 1891 

 with no other equipment than an old theodolite and an opera glass. 

 From 1891 to 1894 he made a rough survey of the southern sky south 

 of deck - 30° which resulted in the discovery of twenty variable 

 stars, four of which are of the Algol type. This large increase in the 

 known number of southern variable stars led Roberts to devote 

 himself more and more to the study of the light curves of known 

 variables. 



