60 LIGHT SCIENCE FOR LEISURE HOURS. 



athwart the line of vision ; and even this he could only 

 do most imperfectly, since his knowledge of the dis- 

 tances of the stars is so limited that he can form but 

 inexact notions of the rate at which the stars are so 

 moving. They may be very far away and moving very 

 swiftly, or they may be at a less (though still enormous) 

 distance and moving with a correspondingly reduced 

 velocity. 



This source of difficulty was very strikingly illustrated 

 when the subject of the stellar motions was treated in 

 connection with the ideas respecting the sidereal uni- 

 verse promulgated by Sir W. Herschel. In the hy- 

 pothesis which regarded the stars as spread with a 

 certain general uniformity through a stratum or slice 

 of space, there was no feature which afforded any pro- 

 mise that by the study of the stellar motions the 

 mysteries of the sidereal universe might be interpreted. 

 The very basis of Sir W. Herschel's own researches into 

 the subject is the vague supposition that it is as likely 

 a, priori that any given star will move in one direction 

 as in another. Later we find Struve presenting his 

 results in the following form : 6 One may wager four 

 hundred thousand to one that a portion of the seeming 

 motions of the stars is due to the sun's motion, and it 

 is an even wager (on pent parier un contre uri) that the 

 latter motion takes place at the rate of between 1 35 and 

 175 millions of miles per annum.' The whole question 

 had become one of probabilities, based on more or less 

 trustworthy assumptions. We cannot wonder greatly 

 that, when Sir Gr. Airy undertook the complete re- 



