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