attainable at the present time. After taking account of all the
disturbing influences exerted on Mercury by the remaining planets, it
was found (Leverrier: 1859; and Newcomb: 1895) that an unexplained
perihelial movement of the orbit of Mercury remained over, the amount
of which does not differ sensibly from the above mentioned +43 seconds
of arc per century. The uncertainty of the empirical result amounts to
a few seconds only.
(_b_) Deflection of Light by a Gravitational Field
image052
In Section XXII it has been already mentioned that according to the
general theory of relativity, a ray of light will experience a
curvature of its path when passing through a gravitational field, this
curvature being similar to that experienced by the path of a body which
is projected through a gravitational field. As a result of this theory,
we should expect that a ray of light which is passing close to a
heavenly body would be deviated towards the latter. For a ray of light
which passes the sun at a distance of sun-radii from its centre, the
angle of deflection ( ) should amount to
image053
It may be added that, according to the theory, half of this deflection
is produced by the Newtonian field of attraction of the sun, and the
other half by the geometrical modification ( curvature ) of space
caused by the sun.
This result admits of an experimental test by means of the photographic
registration of stars during a total eclipse of the sun. The only
reason why we must wait for a total eclipse is because at every other
time the atmosphere is so strongly illuminated by the light from the
sun that the stars situated near the sun s disc are invisible. The
predicted effect can be seen clearly from the accompanying diagram. If
the sun (_S_) were not present, a star which is practically infinitely
distant would be seen in the direction _D_1, as observed front the
earth. But as a consequence of the deflection of light from the star by
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