188
DOCS.
267,
268 MAY-JUNE
1911
Couldn't
you
visit
me
for once?
We have
a room
that
has
nothing
else
to
do
but
wait for
one
of
my
dear friends
to
visit
it .
Besides,
Prague is
marvelous,
so
beautiful
that
it
alone
would
make
a big
trip
worthwhile.
My
wife
and children
are
doing
well.
Both of
the
latter
enjoy truly
robust
good
health
seen very
rarely among
city
children.
It
frightens me a
little
that,
as you
write,
Vero does
not
attend
any school,
even
though
I know how
enterprising
and
gifted
he
is;[5]
I
am
afraid that
in this
way
he will fail to
learn
how to
adapt
himself
to
an
organization,
which
is
so
important
for
every man.
In
fact,
you
too
are a
bit
of
a Gypsy
of
this kind.
What
a
pity!
They
really
need
a man
of
your intelligence
and
your
good
will here!
Wouldn't
you
like to
pitch your wigwam
permanently
here
if,
some
day,
the
occasion
arises?
Then both of
us
would
be
less
lonely.
And
what
agreeable
working
conditions!
All
day long
I
am
in
the institute
and
create.
You
could do
the
same.
Imagine
how nice
it would be if
we
could
study
Grassmann's
Ausdehnungslehre
together.[6]
Cordial
greetings to
all
three of
you
from
your
Albert
In
the
hope[7]
that
all
3
of
you are doing
well
and
feeling
at
home
already
in
your
new
homeland,[8] I
send
you my
best
wishes.
Your
M.
Einstein
268. To
Heinrich
Zangger
[Prague]
7
June
[1911][1]
Dear
Mr.
Zangger,
Thank
you
so
much for
your
kind
visit,
which
was an
extraordinary
joy
for
me.
The
next time
we
are
together
in
the
same
place,
we
should make
better
use
of that than
we
did till now!
Let
me now
write down
for
you
the method for
calculating
the
true
Brownian motion
from
your experiments.[2]
A1
....
An
observed
horizontal
values
r1
......
rn
corresponding
times
h
corresponding height
of
fall
mean velocity
of
fall
mean
square
of the Brownian
motion
v
a
a,
i1
...
rn
are
to be
calculated
such
that the
product
n
n
i
Ch - VTv)2
2
CtTvV e
V2orv
2
ar,
t
2
or
v
be
as
large[3]
as possible.
One takes
the
logarithm
of
this.
Then the
derivative
of
the
latter
with
respect
to
a
and with
respect
to
each of
the
quantities
r
must
then
vanish.
By