422 FRAGMENTS OP SCIENCE. 
mind bad previously imagined such a conception, must, 
we think, be something more than a mere figment of the 
scientific fancy. In forming it, that composite and creative 
power, in which reason and imagination are united, has, we 
believe, led us into a world not less real than that of the 
senses, and of which the world of sense itself is the sugges- 
tion and, to a great extent, the outcome. 
Far be it from me, however, to wish to fix yon immov- 
ably in this or in any other theoretic conception. With 
all our belief of it, it will be well to keep the theory of a 
lumiuiferous ether plastic and capable of change. You 
may, moreover, urge that, although the phenomena occur 
as if the medium existed, the absolute demonstration of its 
existence is still wanting. Far be it from me to deny to 
this reasoning such validity as it may fairly claim. Let us 
endeavor by means of analogy to form a fair estimate of 
its force. You believe that in society you are surrounded 
by reasonable beings like yourself. You are, perhaps, as 
firmly convinced of this as of anything. What is your 
warrant for this conviction? Simply and solely this: your 
fellow creatures behave as if they were reasonable; the 
hypothesis, for it is nothing more, accounts for the facts. 
To take an eminent example: you believe that our 
president is a reasonable being. Why? There is no 
known method of superposition by which any one of us 
can apply himself intellectually to any other, so as to 
demonstrate coincidence as regards the possession of reason. 
If, therefore, you hold our president to be reasonable, 
it is because he behaves as if he were reasonable. As 
in the case of the ether, beyond the " as if" you cannot 
go. Nay, I should not wonder if a close comparison 
of the data on which both inferences rest caused many 
respectable persons to conclude that the ether had the 
best of it. 
This universal medium, this light-ether as it is called, is 
the vehicle, not the origin, of wave-motion. It receives 
and transmits, but it does not create. Whence does it 
derive the motions it conveys? For the most part from 
luminous bodies. By the motion of a luminous body I do 
not mean its sensible motion, such as the flicker of a caudle, 
or the shooting out of red prominences from the limb of 
the sun. I mean an intestine motion of the atoms or mole- 
cules of the luminous body. But here a certain reserve is 
