1921.] 
Farr.—Relativity. 
239 
contraction takes place which just smothers up our possibilities of finding 
out what it is, and it may be anything. And this is what may be called, 
and has been called, the restricted principle of relativity. This principle 
asserts or admits, whichever word you choose to use, that it is impossible 
by any conceivable experiment to detect uniform motion through the ether ; 
and the difference between the attitudes of Lorentz and Einstein is that 
Einstein asserts it and Lorentz admitted it. Einstein asserted it as a work¬ 
ing principle which might perhaps be used to take us further in natural 
knowledge; Lorentz admitted it as an experimental result. The world 
has followed Einstein; and it is for me to-night to try to take you along 
some of the paths which have since been trodden 
In the first place, let us see what influence the recognition of this prin¬ 
ciple has on our ideas of length. We are accustomed to measure lengths 
upon this earth; but a length changes as the earth’s motion through the 
ether changes—-as it certainly does—and so we are immediately up against 
the question, What is length in the absolute sense ? Is there any such 
thing as absolute length ? Upon this earth, since seemingly, whatever the 
earth’s velocity through the ether may be, it changes but little our two- 
foot rules and our yard measures (only 2-J in. in such a comparatively great 
length as the earth’s diameter), it would seem to matter little. But, judged 
from a crude and commercial standpoint, it would not matter at all if the 
changes were thousands of times what the earth’s motion round the sun 
produces—for we should only know it by inference, and not by perception. 
If we conceive ourselves spirited away (as perhaps we shall be some day) 
to another world,, taking our six-foot rule with us—a world which was 
moving through the ether at 121,000 miles a second—we should become 
Lilliputians, contracted in one direction to one-half our present dimensions; 
but we should not perceive it, as the six-foot rule with which we had travelled 
would also be similarly contracted, and would fit with the same exactness. 
It is the philosophical result that absolute length is meaningless, and what 
it implies, that matters, and not (at present, at any rate) any commercial 
consequences of that result. Length, as the formula I have put upon the 
board shows, depends upon our velocity through the ether. Of that we 
have (according to every scientific experiment yet performed to test the 
question) no knowledge whatever. Our universe, therefore, and everything 
that is in it, shrinks or expands in extent according as we assume (as we 
are quite justified in doing) one velocity or another of the earth through 
the ether. Or, let us imagine a school-teacher demonstrating something 
about the properties of a square to a class of interested children—if that 
is not beyond the imagination—and another travelling round and round 
him on the back of an electron at a speed of 121,000 miles a second, and 
catching sight, from time to time, of the figure on the board—the hypo¬ 
thetical board, not this one—he will see the square not as a square 
at all, but as some sort of parallelogram, which is not even necessarily 
rectangular, and the wisdom of the mundane school-teacher will be 
foolishness to him. 
Then, as to time. What does absolute time mean in view of the fact 
that what has been called this “ conspiracy in nature ” exists to block us 
finding out our speed through the ether ? Is time also involved ? The 
reply to that is that it most assuredly is ; but how to make that clear to a 
lay and lady audience is what has been worrying me ever since I was so 
foolish as to consent to address you upon this subject; and I have come 
to the conclusion that the best way is not to try, but to ask you to accept 
