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

TON OF LIGHT.
It may be doubted whether the majority of
the people who went last night TO hear Prof. Iyonah'
felt any certainty about his lecturing or ~ond nave
been much disappointed 10 fnd only bakers dozen
present and to have heard the announcement that the
lecture was postponed. There probably not lees
than 500 people actually in the hall The storm some
what sfted ont the fashionable" element. and bOngh
there still goodly supply OF ladies' the proportion
Of men eminent In science was unusually great Among
them Prof Barnard Of Columbia College, eminent
in discussion principles aud laws OF light 10 whose
presence the lecturer gracefully alluded In the course OF
the evening" and Prof Barker Of Yale College, who has
himself ectured with the highest success on these suD
iecs Men who have investigated Natures secrets seem
not to hesitate before matters Of meteorology
When however, Mr. Hewitt came upon the platform In
stead Of the there general lengthenng OF
faces Everybody expected 10 hear an announcement
OF postponement Mr. Hewitt merely wished TO say that
the sixth lecture would be substituted for the afth and
rice rerd The present lecture they had thought OF post
ponn during the day, in the face Of such an audi
ence postpoDement not TO be thought or Prof.
Tyndal proceeded with his lecture scarcely stopping
for preliminary word of pleased asonishnent al find
ing 81 many present
THE LECTURE.
You have already learned that the word flight" may
be used In two different II may 10 can the i'm
preston made upon consciousness, it may mean tho
physical agent which makes the impression II is with
the agent solely that have to occupy ourselves at
present That agent is the motion OF substance whch
ns all space and surrounds the atoms and molecules OF
bodies. The phenomena to be explained suggest the
assumption the next process is TO ascertain whether
the assumption competent 70 explain the phenomena
To this interstear and interatomic medium certain
definite mechanical properties are sserbed ana We dea
with it as a body possessed Of these properties IN
mechanics have the composition aud resolution OF
forces and Of motions extending 70 the composition ana
resolution Of vibrations We deal with the lunlniferous
with rigid laws of mechanics,
and from the composition resolution, and interference
of its vibrations deduce all the phenomena displayed ~
crystals polarized light.
Let US take as example the crystal OF tourmalne,
| with which we are familiar Let a vibration
cross this crystal obque TO its axis; We have seen OF
experiment that portion Of the light will pass through
| How much determine in this way: Draw straight
line representing the amplitude Of the vibration before
IL reaches the tourmane and from the two ends OF ths
line draw perpendiculars 20 the axis Of the crystal
the distance between the feet OF these two peTpendeu
lars will represent the amplitude Of the transmitted VI
| bration.
Follow me now while I endeavor to make clear 10
| you what occurs when finn Of gypsum is placed
between Nicols prisms. But at the outset let us
establish still further the analogy between the action
OF the prisms and that OF two plates OF tournalne
The plates are crossed and you see that, ny turn
ing the Aim round, place II in position where II mas
| no power TO abolish the darkness. Why ia thst Tue
is that in the gypsum there are two directions az
angles TO each other Wnlcn the Oi
constrained follow and that one OF these drec
tons parallel to OF the axes OF the toumane and
the other parallel the other axis. When this is the
case the film exercises sensible action upon the light
But now the film 10 render its direction OF
vibration obque to the axis; then you see IL mas tue
power demonstrated in the last lecture OF paiahy re-
! storing the light.
lit will be recollected that in the last lecture two
| plates Of tourmaline shown crossed; and where
they we're crossed light passed through On the in
PRODUCTION of Elm OF mica between them light took inc
place Of darkness at their intersection In the present
lecture it shown that transparent film which wond
effect this result II held position ceased to operate
thus around right angles to that postonl
Let us now mount Nicols prisms and cross them
crossed the tourmalne Introducing our him OF
gypsum between them, you notice that In one particular
position the lm has no power whatever over the light
But turn the film little way round, you notice instantly
that the light passes. We have now to understand the
mechanism by which this s effected. =
TUE INTERFERENCE OF RAYS.
First then, we have this prism which receives the
light emergent from the electric lamp, aud which s
called the polarzir. Then have the plate Of gypsum,
and then the prism 'front.' which is called the anayzer
On its emergence from the first prism, the light polar
lied; and this particular its vibrations
cuted in horizontal plane The two directions OF yibra
ton Of tho gypsum are low obhdue 70 the horizon.
Draw : c.1 ;:s@r upon paper 4 re.gffs9t
the directions OF vibration within the gypsum.
Draw line to represent the amplitude Of the vibration
where 10 reaches the gypsum. Let fall from the two
ends OF this line two pependiculars each Of the arms
OF the cross then the distance between the feet Of these
perpendicuars represents the smpudes Of rectan
gular vibrations which the exact equivalents Of the
first single vibration. The polarzed where enters
the gypsum resolved into two others, vibrating at
right angles TO each other.
In one Of those directions OF vibration the ether
more sluggish than in the other and e consequence
the that follow this direction retarded
than the others. You readily imagine that in this
way the system OF waves may get haf wave-lengtk.
indeed any number of half waveengtbs, in advance
OF the other The possibility OF interference here flashes
upon the mind. A little consideration however, rem
ders evident that, long the vibrations
cuted right angles each other, they cannot queneb
each other, matter what the retardaton may ibe.
This brings at 20 the part played by the an.
ayzer It's sole fnncton TO recompound the two ybra
tons emergent from the gypsum. IL reduces them TO
a single plane, where, IL Of them be REGARDED by
the proper amount, extinction occur BIs here, as
in the Of their 1lms, the different lengths Of the
waves Of light come into play. Red will require a
greater thickness to produce the retardat0n necessary
for extinction than blue consequently when the longer
have been withdrawn by interference the shorter
remain and confer their coors the film Of gyp-
sum. Conversely, when the shorter have been
withdrawn the thickness Is such that the longer waves
remain An elementary consideration suiices TO show
that where the direction Of vibration OF prsms aud
gypsum ncoses angle OF As degrees the colors at
their maximum brilancy. When the him lie turned
from this direction, the eolors gradually fade until
the point where the directions OF vibration OF pfisms
and plate are parallel, they disappear altogether
knowledge OF these phenomena il best obtained by
means Of model Of wood pasteboard representing
the planes OF vibration OF the poIarzer and snayzer
and the plane OF vibration the plate gypsum between
them On these planes the waves may be drawn show
I'M fla pc. NJ he nnlaY