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The Photophone . 
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found that all kinds of diverse bodies were rendered tuneful 
by the impadl of an intermittent beam of light. Thin disks 
of wood, glass, metal, ivory, india-rubber, and so on, yielded 
a very distindt note. The apparatus he devised for these 
experiments is illustrated in Fig. 6, where M is a mirror 
reflecting a powerful beam of light from the sun or an eleCtric 
lamp, through a lens, l, and in the path of the beam is 
mounted a rotating wheel, R, perforated round the rim with a 
circle of holes. This wheel adts as a screen to the light, 
except when one of the holes comes opposite the track of 
the beam. The latter then passes on to a pair of lenses, T, 
which diredt the parallel beam toward the surface of a thin 
disk of the material under examination. When the wheel 
is rotated, the intermitted beam of light falling upon the 
disk behind causes it to ring with a musical tone whose pitch 
depends on the number of flashes per minute, and the ear- 
tube attached enables the listener to hear it without inter- 
ference. This musical photophone is in reality a light syren, 
like the air syren of Cagniard de la Tour, in which the puffs 
of air escaping through the holes of a revolving disk emit a 
note. The disk form, though advantageous, is not essential 
to the effedt. Crystals of sulphate of copper, chips of pine, 
even tobacco-smoke held in a glass test-tube before the beam, 
are found to yield a beautiful tone. Nor is it necessary that 
there should be light ; for if the light-ravs be cut off by a 
thin sheet of hard rubber or vulcanite, the invisible heat- 
rays which pass through the opaque screen are capable of 
producing the effedt. Indeed it is still a moot point among 
investigators whether the effedt may not be due entirely to 
the vibratory expansion and contraction of bulk due to the 
recurring blows of the heat-rays. So distinct is the effedt 
that the naked ear held to the disk appreciates it, and even 
the outer ear itself acts as receiver, for when the intermittent 
beam is simply focussed in the aural cavity a faint musical 
note is heard. 
Besides their practical promise, these interesting achieve- 
ments of Prof. Bell have a poetic bearing. We are at once 
reminded of that mystical stone of Memnon which the sun- 
shine made harmonious, and can imagine how the chequered 
sunshine of the trembling leaves is musical to finer ears 
than ours. In Dean Milman’s “ Martyr of Antioch ” the 
god Phoebus- Apollo is invoked by the chorus of maidens as— 
“ Lord of the speaking lyre 
That with a touch of fire 
Strik’st music which delays the charmed spheres.” 
And truly the deep connection between light and music is 
VOL. HI. (third series.) q 
