1921.1] 
Farr.—Relativity. 
235 
I have thus insisted at what you may consider to be some—perhaps 
undue—length upon the ether and the evidence upon which our belief in 
its existence is founded, because perhaps otherwise you might leave with 
the feeling that in postulating it men built upon very flimsy foundations, 
and that it, and hence all other scientific theories, were mostly vain 
imaginings, not backed up by much behind. Such a feeling would be 
entirely a wrong one and very unfortunate, for every scientific theory— 
and this one throughout its long existence of three hundred years is no 
exception—has to run the gauntlet of discussion and experiment in a way 
which would surprise, if they knew it, the promulgators of some other 
theories which find considerable currency in human minds. 
There is, however, just one little cloud, hardly at first perceptible, which 
as time has gone on has overspread the whole horizon. A cloud, should I 
call it, or a faint gleam of light, not at first noticed with much attention, 
but which impressed itself more and more upon the attention as more and 
more became known ? Call it cloud or sunshine as you will, your choice 
will possibly be guided by whether you feel you have a brief in favour of 
the undulatory ether theory as it stands or whether you are prepared to 
follow what appears to be the truth wherever it leads. This last is the 
attitude of every scientific man, and he therefore welcomes difficulties, 
knowing that where a theory seems to be at fault there is there most 
promise of a fertile and illuminating discovery. 
Quite early in the development of men’s notions of this all-pervading 
medium the question be^an to be asked, Does the earth in its journey 
through space drag the ether with it, or does the ether pass freely through 
such apparently solid bodies as the earth, the sun, and the stars ? Is 
there any means of answering such a question, and what is the meaning 
of the answer which experiment gives ? Are we, that is to say, to regard 
the ether as fixed in space, with the earth, the stars, and other heavenly 
bodies passing through it without disturbing it, or do they carry it with 
them as they move, or does their influence die out gradually as the distance 
from them becomes greater ? 
These questions were asked quite early, but not much light was thrown 
upon the answers until Michelson and Morley performed an experiment 
in 1881 which has since become most famous. The experiment was sug¬ 
gested by Maxwell, and was not by any means the first performed in order 
to answer the questions I have asked ; but I think it is the only one I need 
just now refer to. The question they proposed to answer was akin to this 
one : • Imagine two identically equally strong swimmers. Let them both 
start off from a point in mid-stream at the same instant, the one to swim 
a certain distance against the stream and back with it, the other to swim 
the same distance across the stream and back across it. Which swimmer 
will arrive back first at the starting-point ? Simple calculation shows 
that the one who swims across will arrive back first, and from a 
knowledge of the speed at which either swimmer can swim in still water, 
and an observation of the number of seconds late which the one 
swimming up-stream and back arrives, a knowledge of the speed of the 
stream can be obtained. Such a method might not perhaps be the most 
simple or ideal method of arriving at the velocity of a stream. Apart 
from the possibilities of drowning either one or other of these almost 
unobtainable identical swimmers, there is the fact that one can see a chip 
floating down such a stream, and by measuring its speed could arrive at 
that of the stream. But, substituting the motion of the earth through 
