380
CONSTITUTION
OF
RADIATION
The greatest
progress
theoretical
optics has
made
since the introduction
of
the
wave
theory
consists doubtless
in Maxwell's
brilliant
discovery
that
light
might
be viewed
as an
electromagnetic
process.
Instead of
mechanical
quantities,
i.e., deformation
and
velocity
of parts of
the ether, this
theory
introduces into consideration the
electromagnetic
states
of ether
and
matter
and thereby
reduces
optical
problems
to
electromagnetic
ones.
The
more
electromagnetic theory advanced,
the
more
the
question
of whether
electro-
magnetic
processes
can
be
reduced to
mechanical
ones
retreated into
the
background;
one
became
used to
considering
the
concepts of
electric
and
magnetic
field
strength,
electric
space
density,
etc.,
as
elementary
concepts
[4]
that
are
not
in need of
mechanistic interpretation.
The
introduction
of
the
electromagnetic theory brought
about
a
simpli-
fication of the bases of theoretical
optics
and
a
reduction in the
number
of
arbitrary
hypotheses.
The
old
question about
the direction
of
oscillation
of
[5]
polarized light
became
moot.
The
difficulties with
boundary
conditions
at
the
boundary
of
two
media
were
resolved
by
the
foundation
of
the
theory.
There
was no
longer
a
need
for
an
arbitrary
hypothesis
in order
to
exclude* longitu-
dinal light
waves.
The pressure
of
light,
which has
only
recently been
[6]
established
experimentally,
and
which
plays
such
an
important
role
in the
theory
of
radiation,
proved to
be
a
consequence
of
the
theory.
I
will
not
attempt
here
an
exhaustive
enumeration of
the
well-known
achievements but will
rather
consider
a
cardinal
aspect in which
the
electromagnetic theory
agrees
with
or,
more
accurately,
seems
to
agree
with the kinetic
theory.
According
to
both
of
these
theories, light
waves appear
to be
essentially
an
aggregate
of
states
of
a
hypothetical
medium,
the ether,
which
is
present
everywhere even
in
the absence
of
radiation. It
had therefore
to
be assumed
that the
movements
of this
medium must
influence
the optical
and
electromagnetic
phenomena.
The
search for the
laws
governing
this influence
has
caused
a
transformation in the fundamental
views about
the
nature of
radiation, the
course
of which
we
want to
consider
briefly.
The
basic
question
that
arose
in this
regard
was
the
following: Does
the luminiferous
ether take part in the
motion of
matter
or
does
it
move
inside the
moving
matter
in
a
different
way; or,
finally, could it
be
possible
*Translator's
note: The German
text
erroneously
says
"anzuschliessen"
(to
connect)
instead
of
"auszuschliessen"
(to exclude).
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