172 DOC. 38 ETHER AND RELATIVITY
14
SIDELIGHTS
ON
RELATIVITY
the
undulatory
surface
forming
the bound-
ary
between
water
and air
alters in the
course
of
time;
or
else-with the
help
of small
floats,
for
instance-we
can
observe how
the
position
of
the
separate
particles
of water
alters
in the
course
of time.
If the
existence
of
such floats
for
tracking
the
motion
of
the
particles
of
a
fluid
were
a
fundamental
impos-
sibility
in
physics-if,
in
fact,
nothing
else
whatever
were
observable
than the
shape
of
the
space occupied by
the
water
as
it
varies in
time,
we
should have
no
ground
for
the
assumption
that
water
consists of
movable
particles.
But
all
the
same we
could characterise
it
as a
medium.
We
have
something
like
this
in
the
electromagnetic
field.
For
we
may
pic-
ture
the
field
to
ourselves
as consisting
of
lines
of
force.
If
we
wish
to inter-
pret
these lines
of
force to ourselves
as
something
material
in
the
ordinary
sense,
we are
tempted
to
interpret
the
dynamic
processes
as
motions
of these lines
of
force,
such
that
each
separate
line
of