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 ANTIOCH--

'Lord of the speaking lyre,
That with a touch of fire
Strik'st music which delays the charmed spheres.'

Professor Bell has accomplished the curious feat of speaking along a
beam of sunshine 830 feet long. The apparatus consisted of a transmitter
with a mouthpiece, conveying the sound of the voice to a silvered
diaphragm or mirror, which reflected the vibratory beam through a lens
towards the selenium receiver, which was simply a parabolic reflector,
in the focus of which was placed the selenium cells connected in
circuit with a battery and a pair of telephones, one for each ear.
The transmitter was placed in the top of the Franklin schoolhouse,
at Washington, and the receiver in the window of Professor Bell's
laboratory in L Street. 'It was impossible,' says the inventor,
'to converse by word of mouth across that distance; and while I was
observing Mr. Tainter, on the top of the schoolhouse, almost blinded by
the light which was coming in at the window of my laboratory, and vainly
trying to