DOCS.

581,

582

JULY

1918

607

of all for Lorentz

transformations, not[ably]

such linear coordinate

transforma-

tions for which

the

planes

=

C

cut

the

cylinder

into

“cross

sections”

;

thus the

Jo's

can

be defined

by integrals.

For

other

linear coordinate

transformations,

by

contrast,

the

values for

Ja

are

defined

by

the

vector

character

of

the

Ja’s!-

As

you

see,

I

have

gone

through

the

same

stages

as

in

your

letters,

only

that,

owing

to

the

restrictive

precondition

I

placed

at

the

start,

everything

could

be

formulated

more precisely.[9]

I

wonder whether

the restriction

is

necessary

for

the

validity

of

the

theorem?[10]

In

the

case

where

the limitation

is

valid,

do

J1, J2,

J3

in

the

original

coordinate

system

also

vanish,

hence does

the

vector

J

have

the

orientation

of

the

cylinder

enclosing

the

world

tube?[11] I

have not

yet

formed

an

opinion

on

either

of these

points.

I

was

able to

hand

your

e[steemed]

postcard

on

to Mr. Humm[12]

right

away

on Monday.

I

am now

reading Weyl

with extreme

interest.[13]

With best

wishes for

a

pleasant stay

in

the

country,[14]

yours truly,

Klein.

582. From Friedrich Adler

Stein-on-the-Danube, 6

July 1918

Dear

Friend,

It has

been almost

one

year

since

I

wrote

you,

but

I

have been

thinking

of

you incessantly,

for

I

was

occupied

the

whole time

with

relativity

theory.

Now

the

work

is

finally

finished and

I

am

very eager

to know

what

you say

to

it.[1]

I

know

that

you

are so

convinced of the correctness of

your

foundations

that

you

do

not

expect anything

from

further

discussions of it. And

yet

I

would like

to

burden

you

with

the

perusal

of

my book,

for

I

imagine

now

having really

caught

Ariadne’s thread,

leading

to

a

compelling

derivation of

the

necessity

for

a

preferred

reference

system

from

your

transformation

equations.

The

crux

of

the

matter

had

long

been clear to

me,

but

it took

quite

some

effort

to

work out all of

its

consequences

in order

to

make it

convincing

to

others

as

well.

Today

I

would

just like

to know where

I

may

send

you

the

work,

or

for how

long

a

mailing

will

still reach

you

in Berlin.

Now I

have

belatedly

received

the

paper by

E.

Budde in

the

Verhandl. of

1914.[2]

There the

“Einsteinian

optical

clock” is

repeatedly

mentioned. Nowhere

in

your

papers

that

I

know

about

does such

a

thing

appear, though.

It

would be

very

interesting

for

me

if

you

could send

me

the relevant article

or

at least

say

where it has

appeared.[3]

Did

you reply

to

Budde’s

paper,

or was

there

otherwise

any

discussion

attached to it? Considerable difficulties

are

naturally

connected