136 



THE EVOLUTION OF 



the proportion between OS and OZ is accurately pre- 

 served. If the system were uninfluenced by any external 

 body, O would remain absolutely at rest. It is a focus 

 of each of the two elliptical orbits. 



If, in any case similar to those we have imagined, we 

 were to select arbitrarily either of the two bodies, and if 



a. (1862). 



1898. 



1894. - 



b. (1890). 



FIG. 3. Sirius, and relative orbit of companion. 



we were then to measure the distance and the direction of 

 the other from it from time to time, and if we were then 

 to trace the orbit of the second body from these observa- 

 tions, assuming the first to be at rest, we should obtain a 

 curve similar in form to either of the orbits, but larger. 

 Its linear dimensions would in fact be the sum of those 

 of the separate orbits. Such a curve is called the relative 



