386
DOC. 42 SPECIAL AND GENERAL
RELATIVITY
146
Relativity
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
D1,
as
observed from the earth. But
as a
consequence
of the deflection of
light
from the
star by
the
sun,
the
star
will
be
seen
in
the direction
D2,
i.e. at
a
somewhat
greater
distance
from the
centre
of the
sun
that
corresponds to
its
real
position.
In
practice,
the
question
is
tested
in
the
following way.
The
stars
in
the
neighbourhood
of
the
sun are photographed
dur-
ing
a
solar
eclipse.
In
addition,
a
second
photograph
of
the
same
stars
is
taken when
the
sun
is
situated
at
another
posi-
tion
in the
sky,
i.e.
a
few months earlier
or
later.
As
compared
with the
standard
photograph,
the
positions
of
the
stars
on
the
eclipse-photograph ought to appear displaced
radially out-
wards
(away
from
the
centre
of
the
sun)
by an
amount
corre-
sponding
to
the
angle
a.
We
are
indebted
to
the
Royal
Society
and
to
the
Royal
Astronomical
Society
for
the
investigation
of
this
important
deduction. Undaunted
by
the
war
and
by
difficulties
of
both
a
material and
a
psychological nature
aroused
by
the
war,
these
societies
equipped
two
expeditions-to
Sobral
(Brazil),
and
to
the
island of
Principe
(West
Africa)-and
sent
several
of
Britain's
most
celebrated
astronomers (Eddington,
(Cotting-
ham, Crommelin, Davidson),
in order
to
obtain
photographs
of
the
solar
eclipse
of 29th
May,
1919.
The
relative
discrep-
ancies
to
be
expected
between the
stellar
photographs
ob-