302
DOCS.
311,
312
MARCH
1917
This
part
is
slightly
curved
similarly
to
a
small section of
a
sphere’s
surface. We
philosophize
on
how
we
must
construe
the
continuation
of
the
cloth
so
that
an
equilibrium
is
reached in its
tangential tension,
whether
it
is
fastened in
position
at
the
edges,
extends
infinitely,
or
has
a
finite
size
and
is
a
closed unit. Heine has
provided
the
answer
in
a
poem:
“And
a
fool
waits for
an
answer.”[10]
So
let
us
be satisfied and
not
expect
an
answer,
and
rather
see
each
other
again
as soon as
possible
in
acceptable
health in
Leyden!
Sending you
cordial
greetings
and
wishing
you
a
speedy recovery, yours,
A.
Einstein.
312.
From
Willem de Sitter
Leyden,
15
March 1917
Dear
Colleague,
Many
thanks
for
your
kind
letter.[1]
Well,
if
you
do not want
to
impose your
conception
on
reality,
then
we are
in
agreement.
I
have
nothing against
it
as
a
contradiction-free chain
of
reasoning,
and
I
even
admire it.
I
cannot
concur
completely
before
having
calculated with
it,
which
is
not
possible
for
me
to do
right
now.
My paper
in
English
is
now
completely
in
print,
the second
piece as
well.[2]
Unfortunately,
I cannot
send
an
offprint,
because
I
have
not
received
any; prob-
ably torpedoed.[3]
It does not contain
anything
that
we
did
not
already
discuss
in
Leyden anyway.
Just
this: The
simple
fact that
we
can
identify spectral
lines
proves
that in all
stars
and
nebulae,
no
matter
how far
away
they
may be,
the
potential
is
of
the
same
order
of
magnitude as
here.[4]
This
proof
is stronger
than
that
of
the
low
stellar
velocities.[5]
Also,
the fact
that there
is
no
systematic
shift
toward
the
violet in
stellar
spectra
leads
to
the
conclusion
that
an
upper
limit
exists for
the total
mass
of
all stars within
a
given
distance.[6]
Yours
truly,
W. de
Sitter.
I
regret
the
news
about Potsdam
very much,
for
K.
and for Potsdam.[7] deS.