76 FRAGMENTS OF SCIENCE. 



the cause of light. But Newton's notion has not 

 held its ground, being entirely driven from the field 

 by the more wonderful and far more philosophical 

 notion that light, like sound, is a product of wave- 

 motion. 



The domain in which this motion of light is carried 

 on lies entirely beyond the reach of our senses. The 

 waves of light require a medium for their formation 

 and propagation ; but we cannot see, or feel, or taste, 

 or smell this medium. How, then', has its existence 

 been established ? By showing, that by the assump- 

 tion of this wonderful intangible celher, all the pheno- 

 mena of optics are accounted for, with a fulness, and 

 clearness, and conclusiveness, which leave no desire of 

 the intellect unsatisfied. When the law of gravitation 

 first suggested itself to the mind of Newton, what did 

 he do ? He set himself to examine whether it accounted 

 for all the facts. He determined the courses of the 

 planets ; he calculated the rapidity of the moon's fall 

 towards the earth ; he considered the precession of the 

 equinoxes, the ebb and flow of the tides, and found all 

 explained by the law of' gravitation. He therefore 

 regarded this law as established, and the verdict of 

 science subsequently confirmed his conclusion. On 

 similar, and, if possible, on stronger grounds, we found 

 our belief in the existence of the universal aether. It 

 explains facts far more various and complicated than 

 those on which Newton based his law. If a single 

 phenomenon could be pointed out which the aether is 

 proved incompetent to explain, we should have to give 

 it up ; but no such phenomenon has ever been pointed 

 out. It is, therefore, at least as certain that space is 

 filled with a medium, by means of which suns and stars 

 diffuse their radiant power, as that it is traversed by 

 that force which holds in its grasp, not only our 



