DOCS.
249,
250
AUGUST
1916 243
249.
From
Théophile
de
Donder
Brussels, 11
Forestière
Street,
8
August 1916
Sir
and
esteemed
Colleague,
The
lengthy investigations
and innumerable calculations
I
have devoted to
your
theories
oblige me
to
take
a
few weeks
of relaxation:
I
shall
thus
have to
postpone
until later the
calculation in
question
in
my preceding
letter.[1]
I
am
happy
to
note
that
we are now
in
agreement concerning
the
energy
momentum
conservation
law;
I
saw
with
pleasure
that
you
deduce
your [?]
tuA's
from my
10
equations,
which I
[set
for]
the
first time
as[2]
0aBl
=
-dL/dgaB
(ch.
VII,
352
or
Verslag
Amsterd.
of
May
1916)[3]
(1)
and
from my
relations[4]
fgrt
=
-
l)
E
S*
(*.
VII,
353) (2)
equations,
to
which
you
have added
the important
remark
0aßl
=
0aß{L*)Einstein.
May
I
hope
that
it
will
[always
remain
possible]
for
me
to be able
to
count
on
this
collaboration?
Your
manner
of
calculating your
-
tuv's
(arbitrary
g)
The
identification of
my
eqs.
(1)
and
(2)
with
yours
(10
eqs.
of
gravitation)
require very
tedious calculations.
250. To
Michele Besso
[Berlin,
11
August
1916]
Dear
Michele,
Your friend
Ugo
Russi[1] sent
me a
postcard
for
you.
He reports
being
well
and
not
worried. He has
no
fresh
news
about
his mother.[2]
The
latest
was
satisfactory,
though.
His
address
is
Mittergrabern, Oberhollabrunn,
L[ower]
A[ustria],
Barrack
4._[3]
A
brilliant
idea dawned
on me
about radiation
absorption
and
emission;
it
will
interest
you.
An
astonishingly simple derivation,
I
should
say,
the derivation
of Planck’s formula.
A
thoroughly quantized
affair.
I
am
writing
the
paper
right
now.[4]
Your silence calms
me down;
I
hope
all
is well
on
the
way
to
recovery
in
Zurich.[5]
Many
thanks
for
your
last
letter from
the
Planalp.[6]
I
hope
you
were
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