DOC.
42 SPECIAL AND GENERAL RELATIVITY 255
The
System
of Co-ordinates 9
constructions with
rigid rods,
but
by
indirect
means.
If
the
results
of
physics
and
astronomy
are
to
maintain their clear-
ness,
the
physical meaning
of
specifications
of
position must
always
be
sought
in accordance with
the
above consider-
ations.1
We thus obtain the
following
result:
Every description
of
events
in
space
involves the
use
of
a rigid
body to
which such
events
have
to
be referred.
The
resulting relationship
takes
for
granted
that the
laws
of Euclidean
geometry
hold
for
"distanc-
es," the
"distance"
being represented
physically
by
means
of
the convention
of
two
marks
on a rigid body.
1 A
refinement and modification of these
views
does
not
become
necessary
until
we come to
deal
with
the
general theory
of
relativity,
treated
in
the second
part
of this book.