236 The N.Z. Journal of Science and Technology. [jan. 
the ether for that of the rock from which the swimmers dive through the 
stream (or, as we are more accustomed to think of it, for the motion of 
the stream past the rock), and substituting two rays of light for the two 
identical swimmers, the method becomes ideal, and should tell us how 
quickly the earth is moving through the ether, and the direction of that 
motion. For in its annual voyage round the sun the earth at the beginning 
and end of each six months is moving relatively to something fixed, in 
practically opposite directions. If, therefore, the ether is fixed, such an 
experiment should show us the motion of the earth through it; or if the 
earth and ether were together being carried along as are the stream and 
rock upon the surface of the earth in my analogy, still such an experiment 
should give us the value of the earth’s motion relatively to the ether. It 
was calculated that if the earth moved through the ether in its orbital 
journey round the sun a certain result would be observed, which in figures 
amounts to 0-4 of a fringe-width ; but as Michelson and Morley carried 
out the experiment a result not more than one-fortieth of the expected result 
was obtained, and as the experiment was repeated in 1905 by Morley and 
Miller not one two-hundredth of what was to be expected was found. In 
other words, the earth was, or seemed to be, at rest with regard to the 
ether. And yet such could not be so, for the earth alters its motion from 
day to day, and at the end of six months is moving in the opposite direction 
to that which it has at the beginning. A pretty obvious alternative, that 
the ether near the earth moves with the earth, and that the earth’s dragging 
influence grows less and less as it leaves its surface and goes out into space, 
has been considered by Stokes, and the difficulties in the way of supposing 
that such is the case are so great that, whilst it is still just a possible 
alternative, it is with every development becoming a less and less probable 
one. 
Thus the effect of the experiment, repeatedly tried all over the world, 
was to show that if the earth were moving through the ether it could not 
be discovered by this method. But if it could not be discovered, why 
could it not ? The very best scientific men the world possessed turned 
their attention to discovering the answer to that question, and we owe it 
to Professor G. F. Fitzgerald to supply an answer—which may cause some 
of you to think that the scientific men who considered it seriously had lost 
possession of their senses. But indeed they had not, though the suggestion 
which Fitzgerald made is truly most astonishing. Let us go back to the 
more gripable position of the two swimmers diving off the rock in mid¬ 
stream, the one to swim a certain distance up-stream and back, the other 
the same distance across stream and back. I have said that the across- 
stream man should arrive back first, but in the corresponding real experi¬ 
ment the trouble was that, while he should have done so, he just didn’t. 
They both arrived together—identically together. Why ? What happened 
to the up-and-down-stream man to bring him back so quickly ? The 
suggestion which Fitzgerald made was that the up-and-down-stream distance 
was not really so long as the across-stream distance, but that being 
measured in an up-and-down-stream direction it was not measured 
under the same conditions as that across and back. This, put into other 
(perhaps less intelligible) language, is that, of the two paths which the light 
followed, one was up and down in the direction in which the ether was 
moving past the earth, and the other was across and back. Well, 
Fitzgerald’s suggestion was that the distance measured up and down— 
i.e., against and with the ether stream—was not really measured under the 
