136
DOC.
182
OCTOBER
1909
182.
To
Edgar
Meyer
Zurich,
29
October
[1909]
Dear
Mr.
Meyer:
Thank
you very
much
for both
of
your
letters
and for
your investigations
regarding
the
photographic
sensitivity
threshold.
1)
Wood's
curve seems
to
me
only
to
prove
that
the
change
that the
light
at first
produces
on
the
plate
cannot
be
detected
when the
latter
is
developed. [1]
What
we
have here, from the
photographic standpoint,
are
preparatory
processes.
What
strikes
me
is
the
sharp
bend
at
P.
One
would have
expected a
horizontal
tangent.
However, this
might
not
have
been
investigated
with sufficient
accuracy.
The threshold
value that
can
be
seen
here refers
only
to
J
intensity
dt,
and not to
the
intensity
of the illumination
itself.
P time
2)
The
knowledge
that
we are
dealing
here
with
a
combination of several
processes
makes
Schwarzschild's
formula
seem
rather
worthless from
a
theoretical
standpoint.[2]
3)
The
question is
now:
Are
arbitrarily
small
intensities
effective?
Eder[3] states
categorically
that
this
is not
the
case,
as
you
tell
me.
I do
not
believe
that
he
pulled
this
conviction
out
of
his hat. An
experimentalist
like he does not do
that.
Yesterday I
read
an
article in
the
Umschau
that
said
the
same
thing.[4]
4)
One
could
think that
this
proves-if
it
is
true-that intensities that
are
too
small
are
optically
ineffective.
But that
would be
a hasty
conclusion.
Because the
above article
(Umschau
No.
43,
1909)
also
says:
"However,
it
is not
immaterial whether
one
tries
to
obtain
a specific
photochemical
effect
by using
a
single sufficiently long
exposure
or a
series
the
sum
of
which adds
up
to
the
same exposure
time;
such
intermittent
exposure
always
results
in
underexposure,
and it
seems as
if
after each
light
impact
the
plate
finds
time to
recover, to
revert. This
reversion
was
also
established
in
cases
of
sufficiently long
exposure
after
a
lapse
of
several
years."
Hence
it
seems
rather certain that the
inverse
process
occurs spontaneously,
especially
in
the
preparatory
reactions.
Dynamic
equilibrium
thus
occurs,
and it
is
very possible
that
if the
illumination
is
sufficiently
weak,
then
only
nondevelopable
states of
the
elementary
structures
(left
of
P)
will
appear.
Thus,
because of
the existence
of
the
reverse process,
photography is completely
unsuited
for
deciding
the
question
whether
arbitrarily
small
light
intensities
are
chemically
effective.[5] I
think
one
would have to
use
the
photoelectric
effect
(probably
that of
metals
in
the
vacuum
would be
the
best.
Don't
you
think
so
too?