any OQ ANY 
PRESIDENT’S ADDRESS. 19 
the events on a kinematograph screen;—while that great agent of 
continuity, the Ether of Space, will be relegated to the museum of 
historical curiosities. 
In that case differential equations will cease to represent the facts 
of nature; they will have to be replaced by Finite Differences, and the 
most fundamental revolution since Newton will be inaugurated. 
Now in all the debatable matters of which I have indicated possi- 
bilities I want to urge a conservative attitude. I accept the new 
experimental results on which some of these theories—such as the 
Principle of Relativity—are based, and am profoundly interested in 
them, but I do not feel that they are so revolutionary as their pro- 
pounders think. I see a way to retain the old and yet embrace the 
new, and I urge moderation in the uprooting and removal of landmarks. 
And of these the chief is Continuity. I cannot imagine the exer- 
tion of mechanical force across empty space, no matter how minute; a 
continuous medium seems to me essential. I cannot admit discon- 
tinuity in either Space or Time, nor can I imagine any sort of experi- 
ment which would justify such a hypothesis. For surely we must 
realise that we know nothing experimental of either space or time, we 
cannot modify them in any way. We make experiments on bodies, 
and only on bodies, using ‘ body’ as an exceedingly general term. 
We have no reason to postulate anything but continuity for space 
and time. We cut them up into conventional units for convenience’ 
sake, and those units we can count; but there is really nothing atomic 
or countable about the things themselves. We can count the rotations 
of the earth, or the revolutions of an electron, or the vibrations of a 
pendulum, or the waves of light. All these are concrete and tractable 
physical entities; but space and time are ultimate data, abstractions 
based on experience. We know them through motion, and through 
motion only, and motion is essentially continuous. We ought clearly 
to discriminate between things themselves and our mode of measuring 
them. Our measures and perceptions may be affected by all manner 
of incidental and trivial causes, and we may get confused or hampered 
by our own movement; but there need be no such complication in 
things themselves, any more than a landscape is distorted by looking at 
it through an irregular window-pane or from a travelling coach. It 
is an ancient and discarded fable that complications introduced by the 
motion of an observer are real complications belonging to the outer 
universe. 
Very well, then, what about the Ether? Is that in the same 
predicament? Is that an abstraction, or a mere convention, or is 
it a concrete physical entity on which we can experiment ? 
Now it has to be freely admitted that it is exceedingly difficult 
to make experiments on the ether. It does not appeal to sense, and 
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