418
EINSTEIN'S
BOOK ON
RELATIVITY
of the
theory,
Einstein devotes much attention
to
more
foundational issues such
as
the
nature
of the
measuring process
and the
place
of
geometry
in
physical
science.
His discussion of
the measurement
of time and distance with the
help
of
clocks,
mea-
suring
rods,
and
light signals essentially
draws
on
his
very
first
paper
on relativity,
Einstein 1905r
(Vol.
2,
Doc.
23).
The discussion becomes
particularly
clear and live-
ly by
his introduction of the
imagery
of
a
moving
train
to
illustrate the
concept
of
a
moving
reference system-a method that would
be
adopted
later
by many
popularizers.
II
The German
text
underwent various revisions
in
subsequent
editions: Einstein made
some
textual
changes
and added several
appendixes.
Two
of these
appendixes
first
appeared
in
the German
edition;
a
third
one
was
specially
written for the
English
translation
and
was
then included
in
the tenth
German edition
(1920). Two
further
appendixes
were
first
published
in
the
English
edition and did
not appear
in
German
until
1954
when the sixteenth German edition
came
out.[9]
At
the
same
time,
the
English
editions did
not
take into consideration Einstein's
(minor)
textual additions
and
stylistic changes
of the German
editions,
but retained the
text
of the
first
English
edition,
which
was
based
on
the third German edition. Even the last edition
to appear
during
Einstein's
life,
the fifteenth
one
in
English,
which
was
supplemented
by a new
appendix,
and
its
counterpart,
the sixteenth German
edition,
are
not
identical: sub-
stantial differences
are
present,
for
instance,
in footnote references
to
the literature.
Whereas the sections added
in
the third edition and the
first
four
appendixes
con-
tain mathematical details
or
information
on
later
developments,
such
as
the
outcome
of the
light
deflection
measurements, the fifth
appendix
has
a
totally
different char-
acter.
It
is
a
lengthy
discussion of the
"problem
of
space" in
the
context
of
relativity
theory
and
is
of
a more
philosophical
cast
than the
rest
of the book. In the
preface
to
the fifteenth
English
edition Einstein
explained
the
reason
for
its
inclusion: "I wished
to
show that
space-time
is
not necessarily something to
which
one can
ascribe
a
sep-
arate
existence,
independently
of the actual
objects
of
physical reality. Physical
ob-
jects
are
not
in
space,
but these
objects
are
spatially
extended.
In
this
way
the
concept
'empty space'
loses its
meaning."[10]
Although
the
appendix
reveals much about Ein-
stein's ideas
on
space not long
before his death-ideas that had
undergone
a
constant
development
during
his
life-[11]
it
seems
somewhat
out
of
place
in
a popular expo-
sition of
relativity.
[9]After
the fourteenth edition
(1922)
no
new
German editions
were
published
until the six-
teenth.
[10]Einstein 1954,
p.
vi.
The
preface,
which
was
dated
9
June
1952,
was
not
included
in
the
corresponding
sixteenth German edition.
See
Einstein 1917a
(Doc. 42),
note 4,
for the German
text.
[11]See, e.g., Hoefer
1994 for
a
discussion of Einstein's
early conception
of
space.