160 GALILEO AND HIS JUDGES. 



anything hitherto discovered, and one or two modes 

 of explanation were tried in vain. Accident, however, 

 turned Bradley's thoughts in the right direction ; he 

 was one day in a boat on the Thames, and observed 

 that the vane on the mast gave a different apparent 

 direction to the wind, according as the boat sailed in 

 different courses. Here, then, was the solution of the 

 difficulty : it was already known from Eomer's in- 

 vestigations that light moved with a finite velocity, 

 and if so it would naturally produce the same effect 

 as that observed in the boat, or to take an illustration 

 very commonly given, like that which any one finds 

 when moving along rapidly in a shower of rain, in 

 which latter case the rain seems to fall not in the 

 direction it has when one is at rest, but in a direction 

 compounded of tha.t and the one opposite to the 

 person's line of motion. 



Bradley soon drew the correct conclusion, that light 

 acted in precisely the same way upon the Earth as it 

 moved in its orbit, and that the apparent annual 

 displacement of the stars, as detected by him, arose 

 from this sole cause. All the great astronomers who 

 followed him have agreed with his conclusions, and 

 the phenomenon in question, which is called the 

 aberration of light, has conferred a lasting fame on 

 its discoverer. And the remarkable point about it is 

 this, that not only does it give a fresh illustration to 

 the Copernican theory, but it is one of the very few 

 scientific facts that cannot (so far as our knowledge 

 of the subject goes) be explained in any other way. 



