DOC.
21
THEORY OF RELATIVITY
263
already-mentioned bending
of
light rays by
the
gravitational
field of the
sun,
observable
only during
total solar
eclipse,
one
could mention
a tiny
rotation of the
orbital
ellipse
of the
planet Mercury
(40
seconds
in 100
years),
which
can
be
explained by
the
general theory
of
relativity
but
not
by
Newton's
theory
of
gravitation. Finally,
the
general theory
of
relativity
demands
a
slight
displacement
of
the
spectral
lines
produced
on
the surface of the
sun or
the fixed
stars
with
respect
to
the
corresponding
spectral
lines
produced
on
the surface of
the Earth. Observations
have lent
probability
to
the existence of this
phenomenon
but have
not
yet proved
it
with
certainty.