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On the basis of the general theory of relativity, it is found

that the ellipse of every planet round the sun must necessarily

rotate in the manner indicated above; that for all the planets,

with the exception of Mercury, this rotation is too small to be

detected with the delicacy of observation possible at the present

time; but that in the case of Mercury it must amount to 43

seconds of arc per century, a result which is strictly in

agreement with observation.

Apart from this one, it has hitherto been possible to make only

two deductions from the theory which admit of being tested by

observation, to wit, the curvature of light rays by the

gravitational field of the sun,[22] and a displacement of the

spectral lines of light reaching us from large stars, as compared

with the corresponding lines for light produced in an analogous

manner terrestrially (i.e. by the same kind of atom).[23] These

two deductions from the theory have both been confirmed.