Evening star., 1919-09-21

NESS END OF VaH k'hOVOFkdONE
nately Opaque and transparent. One
Of these is nxed in position and the
other is moved bodily in a direction
perpendicular to the strips by the
operation Of diaphragm, to which
1t is rigidly attached, and on which
speech sounds fall The movements of
the diaphragm may be expected, there
fore, to control the obstruction to the
beam Of light In such way that the
intensity Of the emergent beam varies
in accordance with the speech sounds.
The practical objection to this device
is that ordinarily the movements of
the diaphragm are 30 small that it
would be most difficult. II not impos-
sible. to make grids at once SO light.
SO rigid and SO hne as to fulfill the
necessary. conditions. Graham Bell
does not, in fact, claim that this
method has been actually used
His second device is much more
sound from practical. point OF view.
It-relies upon the fact that a vibrat-
Ing. diaphragm 10 continually altering
its CURVATURE; conseau emily. If. pol
ished and interposed In a steady beam
of light It will give rise to reflected
beam which. is Of variable divergence.
Although the total amount Of light
reflected is actually unaltered the
fraction Of It incident at a distance
upon receiver. not large enough to
include the whole beam. will have a
value controlled. by the vibrations of
the diaphragm, The objection in this
case Is that, Of necessity, the dia-
phragm must be large in order to deal
with q large quantity Of light; for
II 1t is placed at q point where the
beam has been concentrated to a To-
Gus the effect Of Its changes of cur
vature upon the divergence of the
beam is negligible. Further, from an
acoustical point Of view. 1t would be
necessary for tho. diaphragm to be
thin and: it would. therefore, not be
suitable for adaptation as a mirror Of
good optical properties."
I'm. this apparatus" says DIR. Ran.
kline. describing his own invention.
the essential point is the substitution
OF the image of grid for the ma
terial grid itself. This substitution
at once surmounts, all the difficulties
inherent IN Graham Bells first pro-
posaL It is no longer necessary to
attempt to construct diminutive rrids
suitable for being mounted upon and
operated by speech receiving dia.
phragms. Both grids are now Hxea
In position and may be Of practically
any size. 1t Is the image of the first
grid which 10 caused by the incident
speech to move to and fro with re-
spect to the second grid. .
The image Of the first grid, in NOV
Ing to and fro over the second grid
In effect opens and closes a sbntter
in the path Of the BEAM"
A curious effect may be obtained
oy speaking loudly. If the voice ia
rot modulated an ordinary word NAV
oe transformed into q soreecil -Four,
and Ave." for example, should he
spoken comparatively softly. "Two"
ana three" need to be said more
loudly.
A special reflector is set up for
use with the sunlight. It is a plain
mirror Of nine inches diameter II
projects the light through. a water
cell to remove most Of the therma,
radiation. Then II passes through n
lens ana the frst grid ana foc,,,..
on the osciliating mirror on the
SOUND box. Tho trumpet, by the way.
10 made Of thick drawing paper. II
was found much superior to tin.
which gives too mstaliio I SOUND.
, Interesting Dossihil,tt.. y.