86 DOCS.

71,

72

APRIL

1915

Hence,

because

the

7uv’s can

be chosen

arbitrarily,

dx'a dx'r

Kr

_

\Zirg

^

dXft

dxv

a/-F

This consideration

evidently

cannot capture

what

you

mean,

because it

is

too

obvious;

but

I

hope

nonetheless

that

by

means

of

the

same

you

will

be able

to

lead

me

toward

understanding

more

easily

what

moves

you

to

object.-

You have

probably

received

my

letter

refuting

your

example.[3]

I

shall

repeat

myself.

My proof

of

the

invariant nature of

8J

fails with such infinitesimal

transfor-

mations in

which

the

guv's

of

the

original system are

constant, because

then the

quantities

Auv

cannot be chosen

freely

but

vanish

altogether.

But this

does

not

apply similarly

to

changing

guv's.

Thus

the

proof

does not fail

generally,

but

only

in

certain

special cases.

Since

the

example

relates to

such

a

special case,

it

proves nothing

about the

validity

of

the

principle

in

general.

I must

even

admit

that,

through

the

in-depth

considerations

to

which

your

interesting

letters have led

me,

I

have become

only

more

firmly

convinced

that

the

proof

of

the

tensor

character

of

uv/-g

is correct

in

principle.

With

cordial

greetings,

yours,

Einstein.

72. To Geertruida de Haas

[Berlin,]

13

Wittelsbacher

St.

[before

10

April

1915][1]

Dear Mrs.

de

Haas,

I have calculated

by

a new

method the

enclosed

curve,

which

your

dear hus-

band

recorded

particularly

meticulously.[2] Although

the

linearity

of

the

damping

term is used

here,

the

legitimacy

of

this

procedure

is

proven

on

the

basis of

the

curve.

It

is

odd

that the

small

displacements

seem so

systematically incorrect;

I

can give

no

explanation

for

it.[3]

The

good agreement

with the

theory is

by

chance,

of

course;

but

it

is

real

inasmuch

as now

any

doubts about

the

theory’s accuracy

are

necessarily

silenced.

I

ask

you

please

to send back the

manuscript

as soon as

possible

so

that

I

can

pass

it

on

to Mr. Scheel.[4] Do

not

forget

also

to

enclose

the observed

curve.

With

cordial

greetings

to

you

and

your

little

children, yours,

A.

Einstein.