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281
Heroes of the Telegraph
·
John Munro
180
'This
crowning
marvel
of
the
electric
telegraph,'
as
Sir
William
Thomson
happily
expressed
it,
was
followed
by
another
invention
in
some
respects
even
more
remarkable.
During
the
winter
of
1878
Professor
Bell
was
in
England,
and
while
lecturing
at
the
Royal
Institution,
London,
he
conceived
the
idea
of
the
photophone.
It
was
known
that
crystalline
selenium
is
a
substance
peculiarly
sensitive
to
light,
for
when
a
ray
strikes
it
an
electric
current
passes
far
more
easily
through
it
than
if
it
were
kept
in
the
dark.
It
therefore
occurred
to
Professor
Bell
that
if
a
telephone
were
connected
in
circuit
with
the
current,
and
the
ray
of
light
falling
on
the
selenium
was
eclipsed
by
means
of
the
vibrations
of
sound,
the
current
would
undulate
in
keeping
with
the
light,
and
the
telephone
would
emit
a
corresponding
note.
In
this
way
it
might
be
literally
possible
'to
hear
a
shadow
fall
athwart
the
stillness.'
He
was
not
the
first
to
entertain
the
idea,
for
in
the
summer
of
1878,
one
'L.
F.
W.,'
writing
from
Kew
on
June
3
to
the
scientific
journal
NATURE
describes
an
arrangement
of
the
kind.
To
Professor
Bell,
in
conjunction
with
Mr.
Summer
Tainter,
belongs
the
honour
of
having,
by
dint
of
patient
thought
and
labour,
brought
the
photophone
into
material
existence.
By
constructing
sensitive
selenium
cells
through
which
the
current
passed,
then
directing
a
powerful
beam
of
light
upon
them,
and
occulting
it
by
a
rotary
screen,
he
was
able
to
vary
the
strength
of
the
current
in
such
a
manner
as
to
elicit
musical
tones
from
the
telephone
in
circuit
with
the
cells.
Moreover,
by
reflecting
the
beam
from
a
mirror
upon
the
cells,
and
vibrating
the
mirror
by
the
action
of
the
voice,
he
was
able
to
reproduce
the
spoken
words
in
the
telephone.
In
both
cases
the
only
connecting
line
between
the
transmitting
screen
or
mirror
and
the
receiving
cells
and
telephone
was
the
ray
of
light.
With
this
apparatus,
which
reminds
us
of
the
invocation
to
Apollo
in
the
MARTYR
OF
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