1892.] on the New Star in Auriga. 617 



cases, a complex system of whirling suns, gigantic in size, and 

 revolving with enormous speed, close about each other. An object- 

 glass, as large in diameter as this theatre, if it could be constructed, 

 would fail to show close systems of stars which the prism easily lays 

 open to our view. 



It is as many as twenty-three years ago since I had the honour of 

 describing in this place the first successful application of this mode 

 of using the spectroscope to the heavenly bodies. The method is 

 now too well known for me to say more than that the change of 

 wave-length or pitch of the light shows itself by a shift of the lines 

 in the spectrum ; towards the blue for an approach, towards the rod 

 for a recession between the light-source and the observer. It is 

 obvious that the prism can take note only of the motions which are 

 precisely in the line of sight. The stars, as seen from the earth, are 

 moving in all directions; the spectroscope selects out of the star's 

 motion, whatever it may be, that part only which is in the line of 

 sight. It is of this component only of the complete motion that we 

 can gain information directly by the spectroscope. 



My original observations of the motion of Sirius were made in 

 1868, and of other stars in the following years, but the advance since 

 then, and especially in recent years, in the improvements of instru- 

 ments and in the use of the sensitive gelatine plate, has made a much 

 higher degree of accuracy in the determination of motions attainable 

 now, than was then possible. To Prof. Vogel is due the working 

 out of a photographic method by which he has now determined the 

 motions in the line of sight of more than fifty stars.* 



This method is applicable not only to the drift of star-systems, 

 but what is of more immediate interest in connection with the new 

 star, to the internal motions within those systems. The simplest 

 case of such systems is where one body only is bright enough to 

 produce a spectrum. Unless the plane of the orbit is across the line of 

 sight the star will have alternate periods of approach and of recession, 

 and the lines in its spectrum will be seen to swing backwards and 

 forwards relatively to a terrestrial line of the same substance in times 

 corresponding to the star's orbital period. A grand example of this 

 state of things was revealed by the discovery at Potsdam of the orbital 

 motion of the bright star of Algol showing that the variation of its 

 light is caused by its being partially eclipsed at intervals by a dusky 

 companion star, the existence and motions of which were thus brought 

 to light. 



* Photographs of the spectrum of Sirius compared with that of iron, and pho- 

 tographs of the spectra of other stars, showing motions in the line of sight taken 

 at Potsdam were thrown upon the screen. 



I wish to express my great obligations to Professor Vogel, Professor Pickering, 

 Professor^ Holden, M. Deslandres, MM. Henry, Dr. Belopolsky, Dr. Roberts, and 

 Father SiJgreaves, for photographs of star-motions and of the New Star, and its 

 spectrum, many of which were specially prepared for this lecture. The photo- 

 graphs not suitable for throwing upon the screen were exhibited m tlie Library. 

 Vol. XIII. (No. 86 ) 2 t 



