sequent work. But these equations
were entangled with the hypothesis of the ether, and with the notion
of motion relative to the ether. Since the ether was supposed to be
at rest, such motion was indistinguishable from absolute motion. The
motion of the earth relatively to the ether should have been different
at different points of its orbit, and measurable phenomena should
have resulted from this difference. But none did, and all attempts to
detect effects of motions relative to the ether failed. The theory of
relativity succeeded in accounting for this fact. But it was necessary
incidentally to throw over the one universal time, and substitute
local times attached to moving bodies and varying according to their
motion. The equations on which the theory of relativity is based are
due to Lorentz, but Einstein connected them with his general principle,
namely, that there must be nothing, in observable phenomena, which
could be attributed to absolute motion of the observer.

In orthodox Newtonian dynamics the principle of relativity had a
simpler form, which did not require the substitution of local time
for general time. But it now appeared that Newtonian dynamics is only
valid when we confine ourselves to velocities much less than that
of light. The whole Galileo-Newton system thus sank to the level
of a first approximation, becoming progressively less exact as the
velocities concerned approached that of light.

Einstein's extension of his principle so as to account for gravitation
was made during the war, and for a considerable period our astronomers
were unable to become acquainted with it, owing to the difficulty
of obtaining German printed matter. However, copies of his work
ultimately reached the outside world and enabled people to learn more
about it. Gravitation, ever since Newton, had remained isolated from
other forces in nature; various attempts had been made to account
for it, but without success. The immense unification effected by
electro-magnetism apparently left gravitation out of its scope. It
seemed that nature had presented a challenge to the physicists which
none of them were able to meet.

At this point Einstein intervened with a hypothesis which, apart
altogether from subsequent verification, deserves to rank as one
of the great monuments of human genius. After correcting Newton,
it remained to correct Euclid, and it was in terms of non-Euclidean
geometry that he stated his new theory. Non-Euclid