li)l-2] on Sir Wi/fi/nn Bcrsrhel. 463 



tluit component of tlie motion of a star which was concealed from 

 Herschel is now known w'ith the greater certainty. Moreover, being 

 ignorant of the distance of the stars, he couUl only express the trans- 

 verse component of motion in seconds of arc. 



A wonderful corollary also results from the use of the spectroscope, 

 namely, the existence of many stars known as " spectroscopic binaries." 

 As seen even with the most powerful telescope such a star is a single 

 point of light, but if the spectral lines are duplicated we know that 

 the source of light is double, and that one component is approaching 

 us and the other receding from us. In this way the orbits and rela- 

 tive masses of these visually inseparable stars are determinable. Tlie 

 number of known double stars, including both visual and spectro- 

 scopic ones, is already large, and Campbell, of Lick Observatory, has 

 expressed his opinion that one star in six is double. Some of them 

 revolve so near to one another and in such a plane that they partially 

 eclipse one another as they revolve, and thus produce a winking light 

 like that of a lighthouse. It would seem that we can now even tell 

 something of the shapes of a pair of stars visually inseparable from 

 one another. But I must not go further into this subject, and will 

 only repeat Arago's saying, that this discovery of HerschePs has " le 

 plus d'avenir." 



It is a figure of speech to refer to the stars as fixed, for a large 

 number of them possess a measurable amount of "proper motion" 

 relatively to their neighbours. The existence of double stars was 

 discovered by the observation of their movements, and thus the study 

 of proper motions is linked to the subject of which I have just been 

 sp:jaking. Some few proper motions had been observed by earlier 

 astronomers, but when Herschel took up the subject proper motion 

 had not Ijeen aci;urately measured in any case. 



If a man is walking through a wond the trees in front of him 

 seem to be operiing out before him, whilst those behind seem to be 

 closing toiieiher. In the same wav if our sun is moving relatively 

 to the centre of gravity of all the stars, the stars must on the ;iverage 

 seem to move away from the point towards which the sun is travelling, 

 whilst they must close in towards its antipodes. These tw^o points 

 are called the apex and antapex of the sun's path. 



Xow Herschel concluded that there was something systematic in 

 the proper motions of the stars, and that there was a point in the 

 constellation of Hercules from which the stars were on an average 

 receding, and that similarly they were closing in towards the antipodal 

 ])oint. The first of these is the sun's apex and the second the antapex. 

 These conclusions were drawn from the motions of comparatively few 

 stars, but the result has been confirmed subsequently from a large 

 number. Moreover, we have now learned by means of the spectro- 

 scope that we are travelling towards Hercules at the rate of about 

 sixteen miles a second. 



During these last few years this grand discovery of Herschel's has 



