Green-Mountain freeman. [volume] (Montpelier, Vt.) 1844-1884, 1873-04-02
Misfellanu.
From Prof. TYNDALL's Lectures on LIGHT
Waves of Sound, Water and Light

In the earliest writings of the ancients vi
find the notion that sound is conveyed b,
the air. Aristotle gives expression to thi
notion. and the great architect, Nitruvius
compares the waves of sound to waves o
water. But the real mechanism OF wav
motion was hidden from the ancients, and in
de.d was not made clear until the time 0
Newton. The central difficulty of the sub
iect was to distinguish between the motion
0f the wave itself and the motion OF the par
.;cle. which at any moment constitute tn,
wave.
Stand unon the seashore and observe tht
advancing rollers before they are distorted
by the friction of the bottom. Every wav'
has back and front, and if you clearl,
seize the image of the moving wave yo'
will see that every particle of water alon!
the front of the wave is in the act of rising
while every particle along its back j
in the act of sinking The particles il
front reach in succession the crest of the
wave. and as soon as the crest is passed they
heo,n 10 fall. They then reacI the iurroV
~ oo,,s of the wave, and can sink no further
Immediately afterward they become the from
of The succeeding wave, rise again until they
resrh the crest, and then sink as before
Thus. while the waves pass onward horizon
tally. the individual particles are slmI ly
Hlrei up and down vertically. Observe {
sea fowl. or. if YOU are swimmer, abandon
vonrs,lf 10 the action of the waves; you ar,
not carried forward, but simply rocked VI
and down. The propagation of wave II
the propagation OF a for Ii, and not the trans
ference of the substance which constitutes
the wave.
The length of the wave is the distance
from crest To crest. while the dists nct
through which the individual particles oscll
late is called the amplitude of the oscillation.
You v i'll notice that in this description the
particles of water are made to vibrate across
tue line of propagation.
And now we have to take step forward
and it is the most important step OF all
You can picture two series of waves proceed
ing from different origins through the same
water. When, for example, You throw
a stone into still water, the ring waves pro
ceeding from the two centres of disturbance
intersecU each other. Now, no matter row
numerous these waves may be, the law hoid'
good that the motion Of every particle OF the
water il the algebraic sum Of all the motions
Unmarried to 1t Ii crest coincide with crest
the wave is lifted to a double height II
furrOw coincide with crest, the motions are
in oppCs ition, and their sum is zero. We
have then still water, which we shall learn
presently corresponds to
WHAT WE CALL DARKNESS
in reference to our present SUBJECT. Inns
action OF wave upon wave Is technically
called interference, a term to be remem-
bered.
Thomas Young's fundamental discovery in
optics was that the principle of interference
applied to light Long prior to his time an
Italian philosopher, GrImaldi, had stated
that under certain circumstances, two thin
beams of light, each of which, acting singly
produced luminous spot upon a white
wall. when caused TO act together, partially
ouenchtd each other and darkened the spot
lhls was statement of fundamental slgnIh-
cauce, but it required the discoveries and
the genius of Young to give 1T meaning.
How he did so, will now try to make clear
TO YOU. You know that AR is compressl-
BILE that by pressure it can be rendered
more dense. aud that by dIlatation it can be
rendered more rare Properly agitated, a
tunlng-fork now sounds IN d manner audi
bic to you all. and most of you know that
the air through which the sound Is passing
l. uarceled out into spaces IN which the ali
II condensed, followed by other spaces in
which the air is rarefied. These condensa-
tons and rarelactions constitute what we cal
wsycs of sound. You can imagine the air o.
room trayersed by series OF such waves
and YOU can imagine a second series sen'
through the same air and SO related u the
first that condensation comedies with con
densation and rarefdction with rareiactlon
The consequence Of this coincidence WoulC
he a louder sound than that produced BY
either system of waves taken sInSly. Bu
YOU can also imagine Il state OF things where
the condensations OF the one system tall up
on tie rarefactiOns Of the other. In thl
case the two systems would completed,
neutrallze each other. Each of them takeI
sjnsIy produces sound; both of them takel
together produce no sound. Thus, by ad
ding sound to sound We produce silence,
Grlmaldi in his experiment produced dark
ness by adding light TO light.
LIGHT Is WAVE MOTION.
The possible analogy between sound an
light htre at once flashes upon the mlnd
Ynupg generallzed this observation. Ii
discovered a multitude OF similar cases, an
determined the precise conditions. On tn
as>umutiun that light was wave motion, al
his exIelIments on interference were ex
planned; on the assumption that light wa
fyipg partic,es nothing was explained. II
tile time Of Huyghens and Euler mt dun
had b, en- assumed tor the transmission {
the waves oil light; but Newton raised tn
objection that, il light consisted OF th
waves of such medium, shadows could no
exist. The waves he cOntended, would BEN
round Opaque bodies and produce the motlo
of light benInd them, as sound turns COZ
ncr.,ra as waves OF water wash round rock
1t was proved that the bending round 1eter
red to by Newton actually occurs, but tina
the infected waves abolish each other b
their mutual interference. Young also es
tabIishtd a fundamental difference betwee
the waves of light and those of sound. CouI
YOU see the air through which sound wave
aIe passing, you would observe every ind
vldual particle OF ar.
o,CILLATING TO AND FRO
in the direction of propagation. Could 20
see the ether, you would also ind every Ind
ydual particle making small excursion
and fro but here the motion, like that {
the water particles above referred to, woul
be across the line of propagation. Th
vibrations of the air are longitudinal, tn
vibrations of the ether are transversal.
THE VELOCITY OF LIGHT.
Every complete vibration of our tunin
fork produces a wave of sound. aDd, as a
sounds travel with the same velocity throug
sr. the more rapid the vibration the shorte
are the sound waves. The pitch OF SCUM
is wholly determined by the rapidity of tt
vibration. as the intensity is by the ampl
tude. The rise of pitch with the rapidity
the impulses may be illustrated by the syreI
which consists OF perforated disk rotatin
over cylinder into which air is forced, an
the end of which is also perforated. Whe
the periorations of the disk coincide wit
those of the cylinder, puff escapes; an