, 1880-12-29

production of the original sounds would be
obtained. This Wa cannot expect yet, but
the results already obtained lead one to
hope that in time even this may be achiev'
ed.
The receiver of the ~phOtophone. as al
present arranged. consists of q large con
cave mirror, which reflects and focuses
the light upon q selenium cell this ii
connected with Q battery, and = couple o.
ordinary telephones are included in the
circuit. The selenium cell is very ingeni
ously adapted by Professor Bell 10 it's pur.
pose. 1t consists of alternate discs Oh
brass and mica the edges of which are
coated with selenium, pared to make it as
thin as possible, whilst yet exposing q
sufficiently large surface 10 the action OF
the light. Any increase of light, falling
upon this selenium cell, lessons its electric
resistance hence The vibrations of the
mirror(caused by the words spoken into the
mOuthpiece by the transmitter), altering
somewhat the amount of light received
upon the cell, reproduce themselves audi.
bly,, by means of the greater or less amount
of electricity thereby transmitted through
the telephone. Both transmitter and
receiver must, of course, be SO supported
as to be free to move, according to the
direction in which the beam has to be
sent or received.
There are many difficulties in the practi-
cal working of this little instrument, but
though entirely satisfactory results have
not yet been obtained, the principle is be-
yond dispute that sound and light can act
upon one another in the described.
Articulate speech has been transmitted by
means of the telephone to o distance Of
some $50 yards, the voice being heard
s {. IS .lk9sgg9flly .ass in alghkia &J hs0i6d
varies in intensity in all unaccountable
manner.
Professor Bell has arrived at many in.
teresting results while experimenting upon
this instrument. He has found that curl
GUS molecular changes take place not only
in selenium, but also in thin surfaces OF
almost any substance; so that they respond
by audible vibrations, to the action OF an
intermittent beam of light. There is o
great difference, however, in the sensitive
ness of the different substances; vulcanite
is one of the best, carbon is very good,
but water is absolutely ~irrespOnsive, and
glass, unsilvered, is also bad. Upon this
discovery, Professor Bell has constructed @
simple form Of p~hotOphOne for transmitting
musical tones.
A beam of light is thrown upon q mirror.
and focused by u lens as before; at the
focus is a disc, perforated round its circum'
ference with numerous holes. From this
disc. which can be rotated so that the
beam passes through a varying number OF
holes, according to the speed OF rotation,
the light passes on to q receiving disc OF
ebonite. from whence the sounds are con
veyed by q tube to the listener. That
these musical s0unds--which are much
lOuder. than the spoken words ARE really
due to the action of light or radiant energy
of some form, may be easily proved, fs
when the beam ia interrupted by means
of a disc of some Opaque body, though the
perforated disc is still rotating, nothing ls
heard at the receiver. No wires are need
cd as conductors between the transmitse1
and the receiver; the beam of light forms
the only necessary connection, and TINS
beam of light, with the simple apparatus
described, has been the means Of convey.
ing distinct musical sounds to q distance Oh
more than o mile. Not even that this
distance is a necessary limit, for there ID
no reason why the sound should not Do
carried as far as the light = be thrown
We have here, in fact, q MsxsscG! Aees0ss0s.
The real cause Of the molecular changes
accompanying this action OF an internie-
tent beam of light upon different supstsn-
ces is not yet certain. 1t appears
probable, however, that tine varyi.e
electric resistances Of selenium are diiectly
due to light; whilst, as with the radlome.
ter, radiant heat 10 probably tne real
source of those molecular changes which
produce the audible vibrations o. vulcan.