27 o IN STARRY REALMS. 



movement of a star is so adjusted as to be either entirely 

 in the line of sight or entirely perpendicular to the line of 

 sight. The movement will generally be of such a character 

 that, to use the well-known language of mechanics, it will 

 have one component along the line of vision and another 

 perpendicular thereto. For the complete determination of 

 the movement of the star it is necessary for us to know 

 both these components, and it will be plain in what sense 

 the spectroscopic method and the telescopic method are 

 complementary ; the latter determines the movement per- 

 pendicular to the line of vision, and the former gives us 

 the movement along the line of vision. Each method just 

 gives what the other is unable to give ; there remains, 

 however, the important difference, that the spectroscope 

 indicates instantly, if I may so speak, the velocity of the 

 body along the line of sight, while the telescope only 

 indicates the portion of the velocity perpendicular to the 

 line of sight by a comparison of observations separated by 

 a long interval of time. 



There is another essential distinction to be noticed 

 between the information conveyed by the spectroscope and 

 that yielded by the older method of observing proper 

 motions. It is a distinct advantage of the spectroscopic 

 method that, so far as its indications are concerned, the 

 distance of the body from the observer is immaterial ; the 

 shift of the line which is observed has a magnitude which 

 depends solely on the relative speed with which the body 

 emitting the light is approaching to or receding from the 

 observer. A star which shows a certain shift of spectral 

 lines due to its velocity would exhibit precisely the same 

 shift were its distance ten times or a hundred times that 



