Iviii Presideufs Address. L-^^S- ^^j 



PHOTOGRAPHY AND ASTRONOMY. 



We have heard so fully from our Secretary, the Astronomer-Royal, 

 about the application of Photography to Astronomy that I shall not 

 enlarge on the subject to-night, but will only refer to one or two 

 points in connection with it. Probably many of you are aware that 

 for several years past regular observations have been made, chiefly at 

 Greenwich and Rugby, to determine the motion of stars in the line of 

 sight, as it is called, i.e., either directly towards us or from us. This 

 is done by measuring the amount of the displacement of some particular 

 line in the star's spectrum caused by a motion of approach or recession. 

 Naturally the amount of this displacement is very small, and owing to 

 this and the tremors in the image caused by the atmosphere, the 

 observations are very difficult and liable to large accidental errors ; 

 indeed, I remember a distinguished astronomer telling me it was the 

 most difficult observation he had ever tried to make. This being the 

 case we have had to be content with results diiffering widely among 

 themselves but probably something like right in the mean of a large 

 number. For example, here are some Greenwich results for the 

 motion in miles per second of Capella : + 4, + 19, + 52, + 38? 

 + 53, — 4, — 55, 4- 42, — 21, + 2, + 30, the mean of all being 

 + 16. 



Quite recently Dr. Vogel, at Potsdam, has photographed the 

 spectrum of this star together with that of an artificial light, and the 

 results of his measures on ten nights vary only from + 14 to -|- 18. 



The agreement between his results and the mean of the previous 

 ones is satisfactory, but what a difference in the accuracy of the indi- 

 vidual measures ! What a gigantic stride in advance are these photo- 

 graphic results over all that had preceded them ! Now at least we 

 may feel sure of arriving at a definite knowledge of the motions of 

 approach or recession of the brighter stars with a fair prospect of 

 .accuracy. 



THE SOUTHERN SKIES. 



The preliminary photographic survey of the Southern Skies which 

 has for the most part been carried out at the private expense of the 

 Astronomer Royal at the Cape, was completed a few months ago, and 

 the positions of the stars are being measured from the plates by Prof. 

 Kapteyn of Groningen. It is satisfactory to know that the plates bear 

 .the test of measurement well, and the result will be a catalogue of all 

 the stars in the southern hemisphere down to 9^ magnitude, the 



