358
THE
RADIATION
PROBLEM
For
example,
if
a
light
source
A
had
emitted
a
light
complex
toward
the
screen
B,
but
it has
not yet
reached the
screen
B,
then,
according to
theories
operating
with retarded forces, the
light
complex
is
represented
by
nothing except
the
processes
that
have taken
place
in the
emitting
body
during
the
preceding
emission.
Energy
and
momentumif
one
does not
want
to
renounce
these
quantities altogethermust
then
be represented
as
time integrals.
To
be
sure,
Mr.
Ritz
claims that
experience
forces
us
to abandon
these
differential
equations
and
introduce the retarded
potentials.
However,
his
arguments
do
not
seem
valid
to
me.
If
one
puts
with Ritz
and
(xf,yl,z]
,t

fi
=
dx] 9dy],dz
(x}
,y*
,z]
yt
+

fi

1
dx] 9dy],dz
41
then
f1
as
well
as f2 are
solutions
of
the
equation
1
d2
f
^

bf
=
p(x
y
z
i)
f3

ad\
+ ^2/2
hence
is
also
a
solution if
a1
+
a2
=
1.
But
it is
not true
that
the
solution
f3
is
a more
general
solution than
f1
and
that
one
specializes the
theory
by
putting
a1
=
1,
a2 =
0. Putting
f(x,y,z,t)
=
f1
,
amounts to
calculating
the
electromagnetic
effect
at
the
point
x,y,z
from
those
motions and configurations of
the electric
quantities
that took
place
prior
to
the instant
t. Putting
f(x,y,z,t)
=
f2
,