DOCS. 74-76 APRIL
1915 89
is
invariant
to
arbitrary
substs.,
thus
also to linear
ones.
Therefore,
according
to
the theorem
in this
case
of
arbitrary
substitutions,
Euv/-g
must
have
a
tensor
character.
Indeed,
Euv/-g
=
const
•
guv
results,
which
is
thus
in fact
a
covariant
tensor.
With best
regards, yours,
A.
Einstein.
75. To Tullio Levi-Civita
[Berlin,]
14
April
1915
Dear
Colleague,
You
pleased
me
greatly
with
your
assentive
postcard.
When
the
occasion
arises to
repeat
the euv
proof,
I
shall
gladly
include the corrections
I
have learned
from
our
memorable
correspondence.[1]
Interest
in the
subject, however,
is
exceed-
ingly
modest
for
the time
being.
It
is
odd how
few
colleagues
feel
the
intrinsic
need
for
a precise
theory
of
relativity. Unfortunately,
the attitude
of
our
nonpro-
fessional
fellow
mortals
is
incomparably
more
peculiar.
So
it
is
doubly gratifying
to
have
come
to know
more
intimately
a man
such
as
you.
I
shall
take
plea-
sure
in
striving
to
allow
our
acquaintance by
letter
turn
into
a
personal
one: one
more reason
at
last
to
cross
the
Alps again
one day.
It
is to
be
hoped
that
our
fatherlands
will
not
rebel
against
each
other
as
well![2]
With
cordial
greetings, yours,
A.
Einstein.
76.
To
Fritz
Weishut
[Berlin,
18
April
1915]
Dear Mr.
Weishut,[1]
I do
not
yet
have
the
offprints
of
the
paper
but
shall send
them
to
you
shortly.[2]
This
is
the
gist:
According
to
the
theory
of
electrons, mechanically, every para
and
ferromagn.
molecule is
a
circular current whose axis coincides with
the
magnetic
axis. From