The Portland daily press., 1898-09-20

Converts Sound Waves Into Waves eff Light
Proves There Is No Such Thing as Act
ual Silence-Work of the Sound-Con
| verting Machine.
New York Sum-
| PORT, A. 6. Webster of Clark Univer-
sity has perfected scientific instrument
which is likely to add considerably to
existing knowledge.of the phenomenon of
sound At the recent meeting of the
American Association for the Advance
ment of Science, Prof. Webster advanced
the question, CHOW loud is q sound?'
When no one3 was able to answer he
brought into view an apparatus which
served as an absolute standard of COM
parison. Hereafter it will be possible to
say just how loud any sound may sbe, not
by guess work, but by means of waves
made by reflected light. Incidentally
Prof. Webster remarked that with this
instrument it my be possible to chase the
echo to its source and find out all about
it; that modified form of the instru-
ment might he installed on shipboard to
be used to denote the approach of other
vessels
Light is the agency that has been called
in to increase the knowledge of sound. 1t
is by means of photography that Prof.
Webster measures all sorts OF noises, from
the roar of an explosion to sounds SO low
thatgthe human ear finds it impossible to
detect them, and which therefore pass
under the name of silence. The device
has proved that there is no such thing as
silence in the ordinary acceptance of the
term, What men call silence includes all
noises below the limit at which the
human ear will register sound, but silence
is really full of sounds that Pass I'M
noticed, except by some delicate perception
like that of Prof. Websters apparatus.
In other words, silence is comparative and
not positive. There are probably few
sounds that can escape this new inven-
ton, for its maker has proved that with 1t
he can detect and photograph the noise
made by draught of air passing through
room.
The mechanism by which Prof. Web
ster's machine records sound ready for
measurements is comparatively simple. 1t
consists of set of movable mirrors and
prisms which act in conjunction with a
diaphragm. One of the principles in
volved in this part of the process is hun
dreds of years old and was discovered ny
Sir Isaac Newton. To illustrate lit sup
pose that light is admitted through PIN
hole shutter into very dark room. In
troduce into the panel Of light any opaque
body, as a knife blade. for example, and
observe the shadow which it casts on a
white screen, and we shall see that the
edges of the shadow are fringed with col
ored light. The light In passing by the
edge or back of razor or block OF
| marble or bubble of air in glass is In
each case affected the same way. This Is
because Ii ght always Is infected. or,
more properly speaking, refraoted, when 1t
passes by the edges of bodies. But 1T Is
rarely observed in ordinary circumstances
because when light comes from various
directions the colors composing 1t overlap
and are reduced to whiteness. Any trans
parent substance Of excessive thinness re-
| fleets brilliant colprs, Examples are seen
| in thin laminae of air ocou plying cracks
/ In glass and ice and the interst1ces be-
| tween plates of mica, also in thin films
| of oil on water and alcohol on glass, but
most remarkably in soapy water blown
| into very thin bubbles.
The same effect Ia produced if lens of
slight oonvexity is laid on plane lens,
and the two after being placed together
by q screw are viewed by reflected light.
Rings of color may be seen ranged around
the point of contact. The smallest rings
are broadcast and most brilliant and each
I .1G% IA firm
In their order. These are commonly
called Newton rings, because Sir Isaac
Newton first Investigated their phenome-
ma, This principle of infection, then,
| and. modification of the instrument
used In producing Newton's rings are
used in the new apparatus I or measuring
sound. Within a small square box Prof.
| Webster has placed several mirrors and
prisms in such manner that Newton 8
rings will be projected against moving
sensitive Photographic plate. On one side
of the box is globe-shaped resonator,
inside of which is sensitive diaphragm
made of very thin glass plate.
| The slightest sound will cause this
diaphragm to vibrate. On the inner side
of the diaphragm is attached very small
circular mirror weighing fraction of a
gramme. When sound, however slight,
enters the resonator or receiver OF the
instrument the diaphragm and conse-
quently the small mirror y1brate back
ward and forward and small beam of
light is sent careening about among the
other mirrors and the prismS and is made
finally to resolve itself into a series of
colorsd fringes. These fringes are project
ed through series of Sits in screen
until by the time the light reaches the
Photographic plate it takes the form of a
waving line This waving line is project
ed on a screen by means of a lantern and
may be viewed if necessary by number
of persons at once, as during the meeting
of the A merican Association for the Ad-
vanceme Mt of Science. So. as Prof. Web
ster iocularIy remarked,a deaf man might
easily distinguish the difference between
the sounds, while blind man could not.
| Some surprising results were shown on
| 4ho conno,, Thns the vibrations nrnduced