56 FRAGMENTS OF SCIENCE. 



emitted by the eye, and not as anything imparted to it. 

 But if light be produced by an agitation of the retina, what 

 is it that produces the agitation? Newton, you know, 

 supposed minute particles to be shot through the humors 

 of the eye against the retina, which he supposed to hang 

 like a target at the back of the eye. The impact of these 

 particles against the target, Newton believed to be 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 propaga- 

 tion; but we cannot see, or feel, or taste, or smell this 

 medium. How, then, has its existence been established? 

 By showing, that by the assumption of this wonderful in- 

 tangible ether, all the phenomena of optics are accounted 

 for, with fullness, 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 

 toward the earth; he considered the precession of the 

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

 plained by the law of gravitation. He therefore regarded 

 this law as established, and the verdict of science subse- 

 quently confirmed his conclusion. On similar, and, if 

 possible, on stronger grounds, we found our belief in the 

 existence of the universal ether. 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 ether 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 planet- 

 ary system, but the immeasurable heavens themselves. 



There is no more wonderful instance than this of the 

 production of a line of thought, from the world of the 

 senses into the region of pure imagination. I mean by 



