DOC.
60
381
that
it
does
not
participate in the
motion of
matter
at
all but
stays
at rest
instead?
To
solve this
problem,
Fizeau
performed
an
important
interference
experiment
based
on
the
following
consideration. Let
light
propagate
in
a
[7]
body
with velocity
V
if the
body
is
at rest.
If this
body,
when
in
motion,
completely
carries
along
its
ether, the light
will,
relative
to
the
body,
propagate
in the
same
way
as
if the
body were
at rest.
Hence
the
propagation
velocity relative to
the
body
will in this
case
also
be
V.
However,
in
absolute
terms,
i.e., relative
to
an
observer
not
moving
along
with the
body,
the
propagation
velocity of
a
light
ray
will
equal
the
geometric
sum
of
V
and
the
velocity
of
motion
v
of the
body.
If the velocities of
propagation
and
of
motion have
the
same
direction
and
the
same
sense,
then
V
abs
simply
equals
the
sum
of
the
two
velocities, i.e.,
Vabs =
V
+ v
.
To
test whether
this
consequence
of
the
hypothesis
of the
completely
co-moving
luminiferous ether is
correct,
Fizeau
made
each
of
two
coherent
[8]
monochromatic
beams
of
light
pass
axially
through
one
of
two
water-filled
tubes
and
then interfere with
each
other.
When
he
then let both the
water and
the light
move
axially
through
the
tubes,
in
the direction of the light in
one
tube
and
in the
opposite
direction in the other
tube, he
obtained
a
shift in
the interference
fringes from which he
could
draw
a
conclusion about the
effect of
the
velocity
of the
body on
the absolute velocity.
It turned
out,
as we
know,
that
the velocity of the
body
does
show
an
influence in the
sense
expected,
but that this influence is smaller than the
hypothesis
of
complete
drag would
require.
We
have
Vabs
=
V
+
av
,
where
a
is
always
smaller than
1. Neglecting
dispersion,
we
get
.
1
a
=
1
~
tt
n£
This
experiment showed
that
the
ether is
not
fully carried
along
by
matter,
i.e.,
that in
general
a
relative
motion
of the ether with
respect
to