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