164
DOC.
38
ETHER
AND
RELATIVITY
6
SIDELIGHTS
ON
RELATIVITY
not
further
reducible. But
the
ether
hypo-
thesis
was
bound
always
to
play
some
part
in
physical
science,
even
if
at first
only
a
latent
part.
[4]
When in
the
first half
of
the nineteenth
century
the
far-reaching
similarity
was re-
vealed which
subsists between
the
properties
of
light
and
those of elastic
waves
in
pon-
derable
bodies,
the
ether
hypothesis
found
fresh
support.
It
appeared beyond question
that
light
must
be
interpreted
as a
vibratory
process
in
an
elastic,
inert
medium
filling
up
universal
space.
It
also seemed to
be
a necessary consequence
of
the fact
that
light
is
capable
of
polarisation
that
this
medium,
the
ether,
must be of
the
nature
of
a
solid
body,
because transverse
waves
are
not
possible
in
a
fluid,
but
only
in
a
solid. Thus
the
physicists were
bound
to arrive
at
the
theory of
the
“quasi-
rigid”
luminiferous
ether,
the
parts
of
which
can
carry
out
no
movements
relatively
to
one
another
except
the
small
movements of
deformation which
corre-
spond
to
light-waves.
This theory-also called
the
theory
of