204 CELESTIAL MEASURINGS AND WEIGHINGS. 



It is to the dimensions of these and similar orbits de- 

 scribed by others of those wonderful bodies, the double 

 stars, about each other, that we have now to turn our 

 attention : thus opening another chapter in the history 



of sidereal mensuration. The mode in which these two 

 elliptic movements, the larger real, and the smaller ap- 

 parent or parallactic, are combined together or super- 

 posed, and the sort of undulating line apparently de- 

 scribed by either star in consequence, will easily be 

 understood by a glance at Fig. 2. 



(29.) Assiduous observation, aided by a powerful and 

 not very complicated system of calculation, has enabled 

 astronomers to assign in a great many instances with 

 considerable precision the true forms of these orbits as 

 distinguished from those in which, by the effect of per- 

 spective (owing to their oblique presentation to our 

 sight), they appear; to state the amount of that obliquity; 



