238
DOCS.
327,
328
DECEMBER
1911
in
my
opinion
the better of
the
two.
First of
all,
he has
an
extraordinary
mastery
of
the
mathematical
apparatus.
It
is
precisely
in this
regard
that
his
paper
about diffraction
on
spheres[9]
is
an
achievement of
the first
order. He
combines
rigorous
criticism with
a good
imagination.
In
addition,
he
possesses
uncommon
mental
agility.
I know
that
he has
command of
all
of
modern theoretical
physics.
From
my
conversations
with
him[10]
I
know
that he
has
a
keen
eye
for
the
important questions
in all
areas
of
physics,
which
I do
not
know
that Keesom
has
(though
that
might possibly
be
the
case).
I
simply
believe
that
you
will
come
to regret
it if
you
were
not to call
Debije
back
to
Holland
while
there
is
still time. On
the other
hand,
I
think that Keesom
can
fully
develop
his abilities
only
if he directs
an
institute,
but
not if he
takes
a
theoretician's
post,
where
he
might
lose
contact
with
experiment, upon
which-as
it
seems
to
me-a
great part
of
his
strength is
based.-[11]
Perhaps
I have
already
said
too
much.
Perhaps
I
am so
partial
to
Debye
because
I
have
made
his
personal acquaintance.
But
I
have
given
you
my
opinion
freely,
and this
is,
after
all,
what
you
asked
me
to
do.[12]
With
best
regards,
I
remain
yours,
A.
Einstein
P.S.
One additional remark about
spectral
lines.
When
n2
(the
refraction
coefficient)
becomes
negative,
then
a
substance reflects
completely (for
those
wavelengths).
This
AXh
h'"h
could
lead
to
small
broadenings
of
absorption
lines toward the violet
h
K
h
ŧ
2
where ü' denotes the contribution made
by
the
kind of
atom
in
question (that
of
the
proper frequency h) to
the dielectric
constant of the
gas.[13]
It
seems
not to
be
totally
out
of
the
question
that
this
effect,
along
with
the other
ones,
shows
up
in
the formation of
solar
spectral
lines. In
particular,
it should be
possible
to
notice this effect in
the
case
of lines in
which
the
"absorption
line"
appears separated
from the
"dispersion
lines"
by
bright
interspaces.
One
would have
to
expect
here
a
broadening
of
the
absorption
core
toward
the violet.
328. To
Robert
Gnehm
[Prague,
19
December
1911,
9:35
a.m.]
Will visit
you
Thursday[1]
morning
in
the
Polytechnic[2]
Einstein