that is capable of being obtained by the delicacy of observation
attainable at the present time. The sole exception is Mercury, the
planet which lies nearest the sun. Since the time of Leverrier, it has
been known that the ellipse corresponding to the orbit of Mercury,
after it has been corrected for the influences mentioned above, is not
stationary with respect to the fixed stars, but that it rotates
exceedingly slowly in the plane of the orbit and in the sense of the
orbital motion. The value obtained for this rotary movement of the
orbital ellipse was 43 seconds of arc per century, an amount ensured to
be correct to within a few seconds of arc. This effect can be explained
by means of classical mechanics only on the assumption of hypotheses
which have little probability, and which were devised solely for this
purponse.
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.
[22] First observed by Eddington and others in 1919. (Cf. Appendix
III).
[23] Established by Adams in 1924. (Cf. p. 132)
PART III: CONSIDERATIONS ON THE UNIVERSE AS A WHOLE
72