The sun., 1872-12-23
PROF. TYNDALL ON LIGHT.
THIRD LECTURE OF THE GREAT
Scientist.


,ion. aud Analysis Of Lisht-Bcautihni ,
Illustrations.
The great hall Of the Cooper institute |
WI. gll,d on saturday night by those of both
.c,,a. who manifested the deepest interest in
,b, lecture Of Prof Tyndall. He said
=, ,ur last lecture we sought to famarlye our
swim De- with the characteristics Of wave motion.
". '.,w q clear distinction between tho motion
cftc wave Itself and the motion Of It's constitu-
on, -t.ticlcs. Passing through water-waves and
a.-..,ts. we prepared our minds for the con
,.ri.on " waves Of light propagatcd through
tl. huiuiferous ether Tho analogy of sound
will ax the whole mechanism in your minds.
Hcro we have vihratory body which
,ritu1trs the wavc-motiou; we have, In
,hc air. vehicle which conveys It. and
We have the auditory nerve which receives
Tire IMPRESSIONS Of the sonurous waves. In the
ease 4 light We have In the vibrating atoms of
the luminous body the originators of the wavc-
notion. we have In the ether its vehicle, while
iha optic nerve receives the impression of the
umiuifcrous waves. We learned also that color
WIS the analogue of pitch, that the rapidity "
atomic vibration augmented, and the length ~
the tthcr-wayes decreased In passing fr I'm the
red to tho blue end Of the spectrum. The fruit
ful principle of interference We also found ap-
plicahlc to the phenomena Of light; and We
learned that, In consequence of the different
lengths OF the ether-waves, they were eNtn-
ruished by different thlcI>nesses OF transparcn,
Glms. tho particular thickness which qucncrcd
one color glowing with the Complementary ones.
Thus the colors Of thin plates were accounted
for
now scrENTfriC THEORIES. ARE FORMED.
But one OF the objects OF our last lecture, and
tni' not the least important, was to Illustrate
IN, manner in which scientific theories
formed. They, in the first place, take their rise
In the desire Of the mind to penetrate to the
sources OF phenomena. This desire has long
been part Of human nature. Ii prompted
Cesar to say that he would exchange his VIC
tories for a glimpse Of the sources OF the Nile
II may be seen working In Lucretius; IL impels
Darwin To- those daring spcculations which OF
have learned that In framing the orics the imagi-
nation does not create but that Il expands,
dminshcs, moulds, and rehnes, as the case may
b., materials derived from the world oil Tact and
observation.
The general facts of magnetism are most sim
ply illustrated by magneti1ed bar Of steel,
commonly called a bar magnet. Placing such
magnet upright upon table. and bringing n
magnetic needle near its bottom, one end VI the
needle promptly retreats from the magnet Will 10
is held quivefing there by some Invisible inill-
ence exerted upon II. Raising the needle
the rapidity OF it's oscillations decreases, be-
cause the force acting upon the needle becomes
weaker. AZ the centre the oscillations cease.
Above tie centre the end which had been
previously drawn toward the magnet. retreats,
aud the opposite end approaches. A we ascend
higher, the oscillations become more violent,
because the force becomes stronger. At the
upper end OF the magnet. as at the lower. IL e
force reaches a maximum, but all the lower half
OF the magnet attracts one end Of the needle,
Sad This ~ Se @ssnec oSe
called polarity, and the points near the ends ~
The magnet IN which the force seems concen-
irated are called lie poles.
what then will occur Il We break this magnet
hsiyc. act as it did when il formed part II the
whole macncty No; each half = In Itself n per
fcci masnet. nqsssessng TIVE poles; Th.s may he
d" mdgnet-the steel ola
LADY's STAYS,
fur example, hardened and magnetized. 1t acts
Lee the magnet. When broken, each half acts
l,e the whole and when these parts are agan
n. Arn we have still the perfect magnet pos
yoar urcakiul To rs utmost limit; you Will he
driven 90 prolong your vision beyond that limit,
trilr polarity az Tesiu ent In The ultimate parl-
ri"s Of the magnet. Each atom Is endowed with
ti- polar force.
Like ali Other forces this force n magnetism
is .m"nahle to mechanical laws. and knowing
l"r ulfSru3n ana inaSmltHae "I thT loree' We
needle near bar magnet, IL takes up de-
] ]%niate p0s,l1n fsat tIn position might be
ei the poles Moving The needle round the mag
there IA definite direction Of the needle and
uu other needle u iron will unsW or as well
a. the magnetic needle, for the needle 90 ron :
iu-edle independently magnetized.
1--we ilacc two or more rods of iron near tie
r4guet tIe action becomes more complex. for
then The non needles are not only acted OF 10y
.le magnet but they act upon each other. And
we pass TO smaller masses of iron-to inpn
'ngs, for exGmple-We find that they act sub
udntlt ltlly ~ the needle-. arranging themseiyes
act Oh.
Placing sheet of paper or OF glass over th ;
sHowEEING IRoS FILISGs UroS THE rArFn,
notice tendency of the niluSs 20 ARRANGE
themselves in determinate lines. They cannot
freely follow this tendency for they are ham
the |
item by tapping the paper each tap release.
their bias. But this is an experiment which can
only lie seen by myself. To enable you to see Il.
take Iair Of small magnets and by a simple
optical arrangement throw the Images df The
magnets upon the screen Scattering Iron filings
over the glass plate 10 which the small magne s
are attached, and tapping the plate, you sec the
arrangement OF the iron nlings In those mag
netic curves which have been 40 long familiar TO
scientific men. have never seen more beautiful
specimens of these curves than those recently
obtained by Prof; Mayer Of Hoboksn. and may
add, In passing, that have here named young
professor from whom i expect consderaDl 0
.hIuss.
day tudt the greater portion of his lntcll~ctual
lie was de ,oted TO bordering over them. llg
iuyested the space through which they run with
hud Of materially; End the probability is
ttl"l the pf"tres' OF science. by epnnectlng the
ether, Will prove these lines OF force as
FARRADAY LOVED TO CALL
q magnetic curves, to represent condition OF
ih s. mysterious substratum Of all radiant acton
tltt now wish 20 occupy your attention; 10 u
their relationship TO theorriic conceptions that
we have now o. consider Nv the action " the
bar magnet upon tie needle we obtain notion
OF a polar force; by the breaking Of the strip 4
uIagnetiled steel we obtain the notion thai
polarity can attach itself 10 the ultimate pHrti-
ties Of matter. The experiment with the lion
hngs Introduces new Idea into the minu the
idea, namely, Of structural arrangement. Every
pair of filings possesses four poles, two Of which
arc attractive and two repulsiSe. The attractive
poles approach, the repulsile poles retreat, the
.onsequence being a certain definite arrange
ment Of the particles with reference TO each
other.
Now, this Idea Of structure. as produced by
polar force opens way for the intellect into n
entirely new region, and the reason esk yuu
now 20 accompany me INTO this region Is, tldi
pur next inquiry relates to the action Of crystals
upon light. Before l speak of this action wish
you tn realize the process OF crystalline archi'
lecture.
LOOK THEN INTO 4 GRANITE QUARRY,
AID spend few minutes I'm examining the rock
II is not of perfectly uniform texture. In Is
rather an agRlomerstion Of pieces which, on ex.
aulndtlOn, present curously defined forrns
You have there what mlneralogists call quartz
!pI have felspar, you have mica. In mineral
oglCal cabinet, where tl ese substances are pro-
served separately you will obtain some notion
vii see there