THE MOVEMENTS OF THE STARS. 283 



shift of the lines of the spectrum through more than a 

 fractional part of the interval between the components of 

 the sodium line. 



To understand the method of procedure it is necessary 

 to recall the appearance presented by the spectrum of a 

 star. It gives us generally the red, orange, yellow, green, 

 blue, indigo, violet, forming the spectrum band so familiar 

 to every one who has ever looked through a prism. From 

 one end of this band to the other transverse lines vari- 

 ously grouped often double, sometimes exquisitely fine, 

 sometimes more or less ill-defined are to be seen. The 

 interpretation of these lines is now well understood. They 

 are due to the presence of metallic or other vapours in the 

 atmosphere surrounding the star. Such is at least the 

 general description of the origin of the majority of the 

 lines. Some are, however, caused by the presence in the 

 atmosphere of the star of other substances besides metals ; 

 there are, for example, numbers of stars of which the 

 brilliant Sirius may be taken as the type, the characteristic 

 feature of which is the presence of a number of lines due 

 to the element hydrogen. 



For our present purpose, however, it is especially essen- 

 tial to observe that the lines are caused by something per- 

 taining to the star itself. We have to invoke their aid 

 for the measurement of the movements of the body in the 

 line of sight. For the purpose of description, I shall take 

 the line known as F in the solar spectrum. This line is 

 also found in a good many stars, and it is known to be an 

 indication of the presence of the element hydrogen. The 

 wave length of this line in air is represented by the 

 number 4860*72, the unit being, as usual, the ten-millionth 



