president's address — SECTION A. 89 



only 0-004", a quantity so small that twenty years ago no one ever 

 dreamt of being able to measure it. 



At the meeting of the Royal Prussian Academy of Science on 

 November 28th, 1889, Professor Yogel'^ stated that he had photo- 

 graphed the spectrum of Algol six times — three in the winter of 

 1888-9 and three times in November, 1889 — and that he found 

 before the minimum the lines in the spectrum of Algol are displaced 

 towards the red, showing that the star is receding, and after the 

 minimum they are displaced towards the violet, showing approach- 

 ing motion, and these facts can only be accounted for b)' the theory 

 that Algol is associated with a dark star, and that tlie two revolve 

 in the plane of the line of sight round the common centre of 

 gravity once in 68"8 hours. At minimum the dark star intercepts 

 some of the light by being on this side of Algol, and the 

 photographed spectrum further justified the conclusion ,that 

 the diameter of the larger body was 1,074,000 miles; of the 

 smaller one, 840,000 miles ; the distance between them, 3.269,000 

 miles; the speed of Algol in its orbit, 27 miles per second; and of 

 the dark one, 56 miles per second, and that the system was ap- 

 l^roaching the earth at rate of 2 miles per second. 



In March. 1891, it was announced that Professor Rowland"^ has 

 accurately photographed the whole of the solar spectrum from D 

 down to the extreme ultra violet, by means of concave gratings. It 

 is the most perfect map of the solar spectrum that has ever been 

 made. He has further proved that thirty-six terrestrial elements are 

 ■certainly present in the solar spectrum, the presence of eight others 

 is doubtful, and fifteen others (incluciing nitrogen, as it shows itself 

 under the electric spark) have not been found in it ; and it follows, 

 he thinks, that if the whole earth wei-e heated up to the tempera- 

 ture of the sun its spectrum would resemble very closely the solar 

 spectrum. 



On August 7th, 1891, M. Deslaudres'* exhibited the results he 

 liad obtained since May in photographing the bright lines of the 

 solar prominences. The negatives show good reversals of the lines 

 H and K, and the first two lines of the ultra violet hydrogen 

 lines. Professor Hale, of Chicago, also in the middle of April 

 obtained the first reversals of the lines H and K by his method. 



The year 1891 will ever be memorable in the annals of 

 astronomy as that in which the great work of a photographic 

 survey of the heavens, which was arranged in 1887 at the Paris 

 conference, was actually bei'un.'* 



The 24in. star camera, the splendid gift of Miss Bruce, was nearly 

 finished in January, 1893, and it had been decided to use it first at 

 the Boyden Observatory, near Arequij^a, under Professor Picker- 

 ing. 



We now come to one of the most surprising results that has marked 

 the application of photography to the wants of the astronomer. 

 Several attempts had been made with more or less success to get a 



