24 PRESIDENT’S ADDRESS. 
true only for corpuscular light,—that the velocity of light is not an 
absolute thing, but is dependent on the speed of the source. With 
corpuscular light there is nothing to observe; with wave light there 
is something, but we cannot observe it. 
But if the whole solar system is moving through the ether I 
see no reason why the relative ether drift should not be observed by 
a, different residual effect in connection with Jupiter’s satellites or the 
right and left limbs of the sun. The effect must be too small to 
observe without extreme precision, but theoretically it ought to be 
there. Inasmuch however as relative motion of matter with respect 
to the observer is involved in these effects, it may be held that the 
detection of a uniform drift of the solar system in this way is not 
contrary to the Principle of Relativity. It is contrary to some state- 
ments of that principle; and the cogency of those statements breaks 
down, I think, whenever they include the velocity of light; because 
there we really have something absolute (in the only sense in which 
the term can have a physical meaning) with which we can compare 
our own motion, when we have learnt how. 
But in ordinary astronomical translation—translation as of the 
earth in its orbit—all our instruments, all our standards, the whole 
contents of our laboratory, are moving at the same rate in the same 
direction ; under those conditions we cannot expect to observe anything. 
Clerk Maxwell went so far as to say that if every particle of matter 
simultaneously received a graduated blow so as to produce a given 
constant acceleration all in the same direction, we should be unaware 
of the fact. He did not then know all that we know about radiation. 
But apart from that, and limiting ourselves to comparatively slow 
changes of velocity, our standards will inevitably share whatever 
change occurs. So far as observation goes, everything will be prac- 
tically as if no change had occurred at all;—though that may not be 
the truth. All that experiment establishes is that there have so far 
always been compensations; so that the attempt to observe motion 
through the ether is being given up as hopeless. 
Surely, however, the minute and curious compensations cannot 
be accidental; they must be necessary? Yes, they are necessary; and 
I want to say why. Suppose the case were one of measuring thermal 
expansion; and suppose everything had the same temperature and 
the same expansibility; our standards would contract or expand with 
everything else, and we could observe nothing; but expansion would 
occur nevertheless. That is obvious, but the following assertion is not 
so obvious. If everything in the Universe had the same temperature, 
no matter what that temperature was, nothing would be visible at all; 
the external world, so far as vision went, would not appear to exist. 
