388 DOCS.
392,
393
OCTOBER
1917
where,
although
r
and
r'
are
independent
of
t, they
are
nevertheless
dependent
on x or
x'.[2] Example:
If in
the
equation
x! +
a
dx
.
dx'
x


x.
=
;
x

1
+
ax' du
du'
one
sets x'
=
0,
one
obtains the
following equation,
x'
=
a,
which
upon
integration
can
be in
agreement
with
u'
=
ß(u

ax)
only
when
one
sets the
integration
constant
=
x'/ß:
x
=
au
+
x'/B,
from which
x'
=
ß(x

au).
Incidentally,
it
is important
to
note that,
in
general,
c
and
c'
are
not
constants,
but
are
themselves functions
of
t;
they
should be
interpreted
as
the
“fourth ho
mogeneous
velocities,”
analogous
to
the fourth
homogeneous
coordinates.
Later,
I
shall
give
you a
complete report
about
this
new
“kinematic
theory
of
relations,”
since
my
current
papers
do not contain
any
sufficient
explanation
of it.
With best
regards,
I
remain
yours.
393. From Gunnar Nordstrom
Leyden, 44
Nannie
Street, 23
October
1917
Dear
Prof.
Einstein,
Cordial
thanks
for
your
kind
letter. It
has
strengthened
my
conviction
that
you really
will
be
coming
to
Leyden.
You
can imagine
how
pleased
Ehrenfest
and Lorentz
were
when
I
told
them
that
your
arrival
is not
improbable.
We all
hope keenly
that
your
health
will
allow
the
trip.
Naturally,
this
time
you
must
have
a
complete
rest
here and not be
taxed
as
much
as a
year
ago.[1]
Ehrenfest
and
Lorentz
share
my opinion, though,
that
this
can
certainly
be
attended
to.
You
will
have such
a
calm and nice time here
that
your sojourn
will certainly
be
favorable
to
your health;
and
you
know
that
you
are
welcome.
Now to
science and
the little
tuv’s.
The calculation
I made,
based
on
formula
(50)
of
your
Annalen
paper,[2]
using
the
approximate
values
(70), again yields