240

DOCS.

244,

245 JULY

1916

him,

is

a

first-class achievement.[3]

I

did

not

mention him

because, frankly speak-

ing,

I

consider him too

old. I

do not know his

exact

age,

but

his

entire

work

actually

belongs

to

an

earlier

epoch.

While Kustner’s research

is

characterized

precisely

by

the

search for

new

avenues,

Müller

is

better

characterized for his

careful

application of

tested

methods

(previously

also

by

himself).[4]

But

even

if

the

type

of

method

is

disregarded

and

success

of

the

application

alone

is consid-

ered

instead,

Küstner’s researches

are

certainly

not inferior

to

Müller’s. In

my

opinion,

Küstner

is

the

man

who

can

be

expected

to

make

Potsdam

move

into

the

forefront: Müller would

perhaps

be able

to

keep

it

at

the

level

it

is.

Yours

truly,

W. de

Sitter.

245. To

Michele

Besso

[Berlin,] Monday.

[31

July

1916]

Dear

Michele,

Your

postcard

made

me

very happy,

as

did

your

prior

letter,

according

to

which

my

wife is

feeling

better

agai.[1]

Do

none

of

you

have

a

clear

insight

into

her

malady?

I

again

firmly

believe in

the

resilience

of

our

friendship.[2]

The “Uncle

Toby”

salutation

is

a

reminder of Sterne’s

novel,

the

main char-

acters

of

which

we

compared

ourselves

to

many

times in

Zurich.[3]

The

rotating

ring’s

field

in

the

proximity

of

the

axis is

found

as

follows.

The

field

in

first-order

approximation

easily

results from

a

direct

integration

of

the

field

equations.[4]

The

second-order

approximation

results from

the

vacuum

field equations to clos-

est

approximation.[5]

The first-order

approximation yields

the

coriolis

forces,[6]

the

second,

the

centrifugal

forces.

That the latter

come

out

correctly

is

self-

evident from

the

general

covariance

of

the

equations,

hence

actually calculating

it out

is

of

no

interest whatsoever

anymore.

This

interest

is

there

only

if it

is

not known whether

rotational transformations

are

among

the

“admissible”

ones,

that

is,

if

the

equations’

transformation

properties

are

not

clear, a

stage

which

has

finally

been

overcome,

thank

heavens.[7]

Heartfelt

greetings

also

to

Anna and

Vero, yours,

Albert.

[6]Recipient’s

appended

note:

“How is

this

supposed

to

work where

the

coriolis

forces

are

independent

of location?”