New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1872-12-27


envs iSsse bands; but they are far better shown by
lhiAcrcular finn which is so worked as 20 be thnneet
the center outward These splendid rings OF hgn' ana
darkness thus produced. 49, - ,~ ,,.
the Engs also seen; but they occur at Doinnsr
portions q the AIM hey are smaller nsn the rnsS
wedgefOrm, which grows gradualy IsicAel H rsd
sSSs 20 s ss 2ssnrrent Sa nRSS' Of LIGHT
and darkness the dark bands occurring at
while the light bands occur between the
ing white light may be now Inferred nasmueh az the
colored rings produced by the white light.
TFhe experiments by which this part Of the lecture
was illustrated very effective The interference
rings IN light very distinct aud the
adherence in size between these rings in blue and in red
light was notable. The iriscolored rings were beautiful
hut there was so much white light with them that they
did not present remarkable brihancy.l
some Of the chromatc effects Of irregular crysta
ration are beautiful in the extreme. Gould introduce
between our NicOls OF glass covered by those
frostferns which the cold weather renders now SO re
irregular capitalization glass plates now to De pre-
sented 70 you, Illustrate what you ngn'
the frosted wndOwpane. And not ony 40 cfystaiilne
bodies act thus upon light, but almost all bodies that
rule orsanc bodies act in this way for this archtec
ture involves Arrangement OF the ether when ln
of she'll for example yields very beautiful eolors IN
polarized light In tree me ether certainly Possesses
hber and were wood transparent, this pecunarity OF
CSpomstc phenomenS like those that you have
But not only do bodies built permanent by nature
strain OF by pressure temporary double-refractiDg
upon uoncrystslllne bodies such as common
PRESSURE BxERTED BY PoLARIzED LIGHT.
When place this bar of wood my knee and seek
10 break It, what is the mechanical condition OF the bar
bends. and its surface is strained Longitude
nay It's surface that next my knee, is long
tudnAly pressed Both the strained portion and In
he eft is
which would render the wood, were II transparent
Misunderstanding Let repeat the experiment with
such bar. BY the dim residue OF light lingering upon
effect upon the light / simply bend the glass bar with
my finger aud thumb keeping its length obique to the
flashes out upon The sides Of the bar
Illuminated the edges most for here the STRAIN ana
sure. portion OF the glass where neither is
This the sOcalled neutral axis of the bar Of
glass aud along IL you dark band, indicating that
the glass along this axis exercises no action upon me
force my finger and thumb, the briancy of the light
Ia augmented.
inserted into press of another kind Introducing the
square between the prlsms IT'S neutrality Is declared
but hardly hold IL sufficiently loosely TO prevent
Its action from maniesting itself Already though the
pressure innitesimal yoR spots of light at the
points where the press with the glass.
now this screen. Instantly the image Of the
S n eaoS BY
bands. Every pair of adjacent luminous spaces is in
opposite mechanical conditions. On one SIDE OF the
dark band have strain the other side pressure
while the dark band MARKS the neutral between
both. / now tighten the wise, and you see color
tighten sul more and the c0l0rs appear as rlch az
de1S The SoCrs vanish tightening Suddenly, they
reappear. From the colors Of q soap-bubble Newton
uniting by the bond Of thought apparently ncongruOu
things From the colOrs here presented TO yon the mag
Dude the pressure employed might be inferred In
deed the NJ Wertheim Of Paris invented an instru
The coors Of posrzed light which exceeded in accuracy
al other Instruments Of the KIND.
IAN these illustrations very successful. The
f.a,r.v .rf r1a VIA
was frightened drew forth much applause from the audi
ence The colors took curvnear forms which added to
beauty They sprang out In these curves from the
points OF pressure. Aferward glass prepared by differ
ences In annealing, to present these eolos III
various figures was exhibited The irregular annealhg
had fixed the particles OF tins glass III state OF pefma
nent strain or torsiOn.
cHRoMATIc CHANGES CAUSED By HEAT.
You know that bodies expanded by heat and con-
tracted by cold II the heat be applied with . uni
fornity local strains pressures come into play
but if one portion OF solid be heated and Others not,
the expansion OF the heated portion introduces strains
preSures which reveal themselves under the scru-
tiny OF polsrzed light. place this square OF common
line but it exerts no action on the pOlarzed light
hold II for moment the flame Of spirltlamp
reiutroducinR II between the NcOls light flashes
upon the Here, in the case Of mechanical
action yOu have spaces OF strains divided BY neutral
more symmeircaly. This small square glass per.
U1re is introduced Placing the square between HE
prisms heating the copper the heat passes by
duction along the 20 the glass, through which 1t
spreads from the center Outwards You beautiful
black inclosng four luminous quadrants growing
MP ana becoming gradually black by comparison upon
the screen. Ana Ia the OF pressure produced
eOOrs mere also, By the proper application Of heat
gorgeous chTOmatc effects may be produced Ana they
NAV be rendered permanent by first heating the glass
sufficiently, aud then cooling IT, so that the chilled mass
e. remain a state OF strain and pressure. Two
re examples will illustrate this point The colors you
quite rich as those obtained 10 the Of
Aad now we have to push these considerations TO a
nDa illustration. POlarZed light may be turned to
count in analyZer Of molecular
ditOn. hold in my hand strip Of glass six feet long,
inches wide, and quarter OF an inch thick. Hold
sweep over OF It's halves wet wooden rag you hear
sound due 10 the vibrations OF the glass What
the condition OF tnat glass while the sound IS heard
THIS its halves lengthen and shorten in quick
ces5on It's ends therefore, are III state Of quick
vibration but what the condition of the centsT Here
the from the two ends alternately meet and re-
kresi their opposing actions the glass at the
center kept mOtOness but on the other hand 1t is
here alternately strained aud compressed The state Of
propagation Of pulse illustrated in for
mer lecture. Here is OF spots, which By simple
mechanical cOntrvance are made 10 yOrate 10 and tO.
You notice the terminal dots have the largest amplitude
ybr8tOu, while those at tue center alternately
crowded together aud drawn apart The condition Of
tins soundng strip OF glass here correctly represented
s. kstck IA 81 M isssyss
IA diagram is given Of the strip Of glass with eQuids