DOC.
58
DISCUSSION
OF
SIEDENTOPF
559
Published in
Physikalische Zeitschrift
10
(1909):
779-780.
Discussion
following Siedentopf
1909,
held
on
20
September
1909,
published
10
November
1909.
[1]
In his
talk,
Siedentopf 1909,
which he
ac-
companied by
slides and demonstrations,
Sie-
dentopf
discussed various methods
for dark-field
illumination in
ultramicroscopic
observations.
He
emphasized
the
advantages
offered
by
the
new powerful
dark-field
condensers,
produced
by
Zeiss, for various
applications, including
the
analysis
of
Brownian motion.
[2]
Heinrich Rubens
was
Professor
of
Experi-
mental
Physics
at
the
University
of
Berlin.
His
research interests included
questions
of
optics
and
black-body
radiation.
[3]
For
an
account
of Perrin's
experiments,
see
the editorial
note,
"Einstein
on
Brownian Mo-
tion,"
§
VII, pp.
219-222.
[4]
Siedentopf
was
director
of
the Zeiss
com-
pany's microscopy
division. It
appears
from his
earlier
publications
(see,
e.g., Siedentopf
and
Zsigmondy
1903) as
well
as
from
a
remark
in
Einstein
1906b
(Doc. 32), p.
371,
that he had
earlier
been interested
in the observation
as
well
as
in
the
interpretation
of
Brownian motion.
[5]
In his
lecture,
Siedentopf
had earlier
pointed
out
that "finally, dark-field
condensers
of
high intensity are
also
appropriate
for
high-
speed photography
of
rapid microscopic pro-
cesses.... Photographs
of
Brownian molecu-
lar motion
are particularly
interesting
for the
physicist" ("[d]ie
lichtstarken Dunkelfeldkon-
densatoren
eignen
sich schließlich auch
gut
zur
Momentaufnahme
schnell ablaufender mikro-
skopischer
Vorgänge....
Für den
Physiker
sind
besonders interessant Aufnahmen der
Brownschen
Molekularbewegung" (see
Sieden-
topf 1909,
p.
779).
Photographs
of
Brownian
motion had been taken
previously by
Henri
(Henri
1908)
and
by
Seddig (Seddig
1907).
[6]
Einstein
was probably referring
to the
mea-
surements
of
Henri and
possibly
also to
some
of
the
experiments performed by
Perrin and his
group.
The
cinematographic
studies of Henri
(Henri
1908)
had
earlier
been criticized because
the
large
quantity
of
light
used for
obtaining
photographs prevented satisfactory
control
of
the
temperature
(Cotton 1908).
A
more
careful
control
of
the
temperature was
achieved in the
experiments on
the
displacement
formula that
Perrin and his coworker Dabrowski
published
in
1909 (Perrin
and
Dabrowski
1909;
see
also
Per-
rin
1909b,
p.
79).
[7]
Max
Seddig was
at
that time
at
the Univer-
sity
of
Marburg;
he later became Professor
of
Physics
at
the
University
of
Frankfurt. His
re-
search interests included scientific
photography.
[8]
In his work
on
the
temperature dependence
of
Brownian
motion,
Seddig
had-in
spite
of
us-
ing
short
exposures
for his
photographs-en-
countered
serious difficulties in
measuring
the
correct
temperature
of the
liquids
he
studied;
this
was one
of
the
reasons
for the lack
of
agreement
between his observations and Einstein's
predic-
tions
(see Seddig
1908).
[9]
See
Svedberg
1906a.
[10] Svedberg
described the
trajectories
of
the
suspended particles as
"sinusoidlike"
("sinu-
soidähnlich")
(Svedberg
1906a,
p. 854).
Sved-
berg's
work
on
Brownian motion had been dis-
cussed
by
Einstein
(see
Einstein 1907c
[Doc.
40])