56 FRAGMENTS OF SCIENCE. 
emitted by the eye, and not as anything imparted to it. 
But if light be produced byan 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 conclusiveuess, 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- 
arv 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 
