320 THE PRESIDENT S ADDRESS. 



that I shall be in order if I state, without occupying too much of 

 your attention, that successful observations made on this princi- 

 ple, have been systematically carried on for many years at the 

 Eoyal Observatory, Greenwich, and at several other observatories. 

 One of the most interesting of the results derived from them is 

 an indication of the probable cause of the periodicity of some of 

 the variable stars, such as Algol, the second brightest star in 

 Perseus, and the subject is at this time creating a great interest 

 among astronomers. It has also been recently pointed out by 

 Prof. E. C. Pickering, that some very remarkable evidences of 

 the periodic doubling of the line K are shown in the photo- 

 graphed spectra of £ Ursae Majoris, /5 Aurigse, and a few other 

 stars. This doubling of the line can only be satisfactorily 

 accounted for by assuming that each of these stars, though 

 apparently single, is really double, of which the components are 

 nearly of equal magnitude, but too close to have been separated 

 as yet visually, on account of their immense distance from the 

 earth. A possible explanation of this remarkable phenomenon 

 has been suggested, by inferring that as the two stars revolve 

 round their common centre of gravity, separated from each other 

 by many millions of miles, they are sometimes moving perpen- 

 dicularly to the line of sight, when the lines of the combined 

 spectrum are seen in their true positions ; but when one com- 

 ponent is approaching the earth all the lines in its spectrum will 

 be displaced towards the blue, while in the other component 

 receding from the earth, the lines will be displaced by an equal 

 amount in the opposite direction, or towards the red end of the 

 spectrum. At such times, therefore, the line, instead of appear- 

 ing single, will be separated into two. In the spectrum of the 

 star £ Ursse Majoris, this doubling of the line K reappears after 

 regular intervals of fifty-two days. 



As a further illustration of the interesting nature of these 

 researches, I may mention that, by measuring the amount of 

 displacement of the hydrogen line p in the spectrum of Sirius, 

 Dr. Huggins found in 1868 that this, the brightest of the fixed 

 stars, was receding from the earth in the line of sight, at the 

 rate of 29*4 miles per second. Prom more recent measures, 

 made principally at the Eoyal Observatory, the velocity of this 



