316
DOCS.
325,
326
APRIL
1917
devote all
that
much time to
our
difference of
opinion,[4]
which
is
only
a
difference
in
creed,
so
to
speak,
since it would not be
fruitfully spent. We
should
see
the
possibilities
without
wishing.
Of
course
you
can
quote my formulation, provided
that
my
rendition
was
not too
vague
and
imprecise.[5]
This
you
can
judge
for
yourself,
of
course.
I still do not
understand the
remark
about the
manifold
you
considered,[6]
ds2
E
dxj
Exl

according
to
which
the
singular
behavior in finitude
is
only
apparent,
caused
by
the
choice of coordinates.[7]
But
your
new
article[8] will
clarify
this
for
me.
In
any
case,
one
thing
stands.
The
general
theory
of
relativity
allows
the
addition
of
the
term
Aguv
in the
field
equations.[9]
One
day, our
actual
knowledge
of
the
composition
of
the
fixed-star
sky,
the
apparent
motions of
fixed stars,
and
the
position
of
spectral
lines
as a
function of
distance,
will probably
have
come
far
enough
for
us
to be able
to decide
empirically
the
question
of whether
or
not
A
vanishes.[10]
Conviction
is
a good mainspring,
but
a
bad
judge!
Cordial
regards, yours,
A.
Einstein.
326.
From Otto
Neurath[1]
Vienna
XIX, 6
Billroth
Street, 15 April
1917
Highly
esteemed
Professor,
I
thank
you sincerely
for
the
kind note
you
sent
my way
last
fall.
Forgive
me
if
I
bother
you
with
an
inquiry
connected
to
my
research.
For
a
long
time
I
have been
contemplating
the
idea
of tracing
the
parallel
course
of
development
of acoustics and
optics.
This
seems
to
me
to
be
a
fascinat-
ing
task
in
more
than
one respect.
As
you
know,
for
a
time,
optics
and
acoustics
followed approximately
the
same course
of
development;
in
particular,
the
idea
of interference
appears approximately simultaneously
in
both
fields.
They
have
undoubtedly
influenced each
other
as
well.
I
should be
sincerely obliged
to
you
if
you
would do
me
the
great
favor of
informing
me
whether
you
know of
anything
related
to
this
question
or even
just
of
a more or
less
instructive
contribution
to
the
history
of interference in
acoustics. Whereas I
have found with relative
ease
valuable
essays
in
the
optical
literature,[2]
all
that
has
come
into
my
hands
up
to
now on
acoustical interference
is
fragmentary
and
scattered.
Before
I
set
about
searching
out
the
essentials
from
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