342
DOC.
353
JUNE
1917
353.
From Erwin Freundlich
Neubabelsberg
Roy.
Observatory,
17
June
1917
Dear
Einstein,
In
the
following
I
would
like to
sketch
my
research
plans briefly
once
again.[1]
The
two
most important
experimental projects
I
would
like to
work
on
in
connection with
your
theory
of
gravitation
are:
1)
Verification
of
the
deflection
of
light rays
in
a
gravitational field.
2)
Verification
of
the
relative shift
of spectral
lines in
gravitational
fields of
differing intensity.
As
concerns
the
execution of
both
projects, essentially
no
special
instruments
are
required,
rather
only large
astronomical
telescopes
with
the latest
accessories,
such
as
those
available at
the
Babelsberg,
Berlin,
Roy.
Observatory.
I have
merely
sought
to
refine details of the
measuring
instruments
and
the
observation
methods
employed
in
processing
the
observation
data
gained.
Thus
at
the
beginning,
the
research work will
cover
the
theory
underlying
the
new
measurement
methods
and the
efficiency
of
the
observation methods.
1)
The first
project,
that
is,
the
investigation
on
the
deflection of
light rays
in
gravitational
fields, can
be carried out
by photographing
the
background sky
during
a
total
solar
eclipse
and
comparing
the stellar distances
on
such
plates
with
corresponding
shots of
the
same
region
of
the
sky
obtained
some
months
before
&
after
the solar
eclipse.
For
the
expedition
undertaken
by
Dr. Zurhellen and
me
in
1914,
which
yielded
no
results
because
of
the outbreak
of the
war,[2]
we
had devised
a
series
of
im-
provements
toward
obtaining
sufficiently precise
data,
which
are
of
some conse-
quence
to
astrophotography
and
the
examination
of
which would be
important
preparatory
work for
the
coming unusually
favorable solar
eclipse
in
1919.
In order
to
become
independent
of
the
rare
moments of
total
solar
eclipse
in
studying
these
problems, however,
I
have also focused
my
attention
on
the
following
two methods:
a) The exposure
of
stars
in the proximity
of
the Sun in daytime.
My
initial
efforts in this direction could never be taken up practically, since
neither
funds
nor
instruments
were
made available to
me.[3]
The fact
that
such exposures have
already been
tried
out with success in England by
Mr.
Lindemann a
short
while
ago proves
that
this method
is
not
without
prospects.[4]
b)
Photographic
exposures
of
stellar occultations by
Jupiter
using Kapteyn’s
parallax
method.[5]
I
have described this method in more detail in an
unpub-
lished
paper
“Über
die
Möglichkeit und
die
Methoden,
welche die
Astronomie