326 THE PRINCIPLES OF SCIENCE. 



3 . At what velocity ? 



4. Is this velocity variable or uniform ? 



5. If variable, according to what law ? 



6. Is the direction uniform \ 



7. If not, what is the form of the apparent path ? 



The successive answers to such questions in the case of 

 certain binary stars, have afforded a proof that the 

 motions are due to a central force coinciding in law with 

 gravity, and doubtless identical with it. In other cases 

 the motions are usually so small that it is exceedingly 

 difficult to distinguish them with certainty. A coincidence 

 of motions in some constellations has been pointed out 

 by Mr. Proctor, and the parallactic effect due to the sun's 

 proper motion has been surely detected ; but the time is 

 yet far off when any general results as regards stellar 

 motions can be established. 



The variation in the brightness of stars opens an un- 

 limited field for curious observation. There is riot a star 

 in the heavens concerning which we might not have to 

 determine 



1 . Does it vary in brightness ? 



2. Is the brightness increasing or decreasing ? 



3. Is the variation uniform, that is, simply proportional 

 to time 1 



4. If not, according to what law does it vary ? 



In a majority of cases the change will probably be 

 found to have a periodic character, in which case several 

 other questions will arise, such as 



5. What is the length of the period ? 



6. Are there minor periods within the principal 

 period ? 



7. What is the form or law of variation within the 

 period ? 



8. Is there any change in the amount of variation ? 



