National Republican. (Washington City (D.C.)) 1872-1888, 1872-12-09

Ills Third Lecture-Lucid Explanations aud
brilliant Experiments. ...

brilliant Experiments. ..
SATURDAY evening last, Professor Tyndail de-
liyerrd the third in his series Of lectures on
light Lincoln hall was again crowded, and by
an audience emlnently distinguished for lts cul.
tuFe and high social position. The main subject
Of his lecture was the action Of crystals upon
light. lie spoke nrst Of the architecture Of
crystals themselves, and how, in most extra
ordinary manner, they are built up, and said we
must fall baCksfOt explanations on conceptions
drawn from the world OF senses HIS nrst eX.
pertinent was with the magnet needle. lie
passed magnetic needle suspended above a bar
magnet, which was repelled At one end and at
tracted at the other while o. the centre Y was
neutral. This was TO tamillarize the audience
with the idea Of polarity.
have one as an attractive and the other as a re-
pusiye magnet7 No; hut two perfect magnets
Then followed the second experiment, with
magnetic strip Of steel tested by o suspended
needle. II was broken and tested again, and he
said q might 20 on %n, Impossible oS to dlylae ,,
the division ill the NJ ms Of the steel were con-
ceiyed a. possessing distinct poles. He hung
some Iron filings upon plate Of glass beneath
which was magnet, and then threw the image
upon the screen. As the plate was Jared the
nilnga arranged themselves In magnetic curves
around the poles. In this there was inst the
idea Of polarity, and second that Of special striC
ture produced by the action Of polarity on par
tlClcs. by this crystals are built MP, mOlccule by
molecule. Crystalline forms are Of lnnnite va
riety. A solution Of chloride OF ammonium was
spread upon glass plate and placed In the cleo
trio microscope. When the solution had sum-
Cently evaporated capitalization took place.
spreading fern-llke structures upon the screen:
Next, a CELL containing solution OF acetate Of
lead was placed In the microscope. On sending
current OF electricity through the two ter
mlnals dipping Into the liquid, beautiful crystals
Of metallic lead were projected upon the screen.
Now, the ether, the same which vibratea In the
interstea spaces IL implicated, and TO Illustrate
thla Prof Tyndall took plate of the crystal
tourmallne reflected horizontally and vertically
fFnm n hlnFE mirror In nOaltlOO the Image
Of the tourmaline was black, In the other u was
transparent. The light passing through the
touTmallne, being reduced to horizontal plane,
could not he rejected when the mirror was so
placed as TO reflect only light vibrating In YER
iiral plane. And again, when the image Of tour.
mallne, reflected upwards, was black, the crystal
was turned ninety degrees, and bright image
thus produced, for the mirror was then In pO-
sitiOn to reflect light vibrating In the same plane
as that passing through the tOurmalne. Iceland
spar was then placed over the dot, and the
two Images Of the dot seen. The spar was turned,
and the farthest image OF the hot, an eNtraOrdl-
nary ray, TevOyed around the nearest image Of
the dot, an ordinary ray,
refracting spar, which showed an Image OF car
hon points from the electric lamp. pom the
screen one image reyolred around the other on
turning the spar. The extraordinary image re-
VOWED around the ordinary Image.
The next was y circular beam OF light split
into two by means Of the spar and reelected
horizontally aud vertically IN one position; the
ordinary ray was quenCcd and the extraordinary
one WAS reflected In the other position Of the
mirror the extraordinary ray was quenched and
the ordinary reflected.
through the spar. The light being thus reduced u
vibrations OF one plane, only one Image was pro-
jected upon the screen end.
Again, when light was passed through, the
pOarizlng prism with the latter In an Obllque
direction, some light vibrating In both planes
was transmitted through the spar, and two
images were obtained,
Tbe ncSt vac an Illustration Of polarizatIon hy
rejection, consisting Of reflected beam from
electric lamp, with glass mirror, (hcld as near
as possible In the polarlzlng angleD through Ice
land spar; the rays being reduced 10 vibrations
Of a single plane, by the mirror only one image
was projected upon the screen. Then followed
an Illustration OF pOlarlzed light by transmission
and by reflection; image Of tOurmallne was re.
nected nrst from 4 single plate Of glass, and q
part of the light also transmitted upon the
screen. As the number Of plates were Increased
one image was black and the other transparent
And the next was an image Of tourmallne split By
passing through a refactlng spar, one image
bright and the other dark upon the screen. On
turning the tOurmhllne around point Us degD Is
reached where light, vibrating In both planes, i
transmitted, and two transparent images are
thus obtained. On completing revolution Of
90 degrees the image that was last transparent
became dark, and vice versa.
Two tOumallne plates were projected upon
the screen; when they were parallel, one Oyei
the other, their image was transparent; on cross
ing them their place OF JunctuTe was perfectly
black. The Introduction Of piece Of mica be,
tween them instantly removed the blackness; by
rendering the light passing through the iirsi
tOumaline OF the same vibrating plane az light
that could pass through the second one ex
planned the Nlcol prsms; the largest made be
longs TO william SpottlswOOde, esq, of London
The prism nearest the light Is called the analLer
the prism farthest from the light, the anallZer
showed that light, after having passed through
the first prism, could he quenched or transmitted
by the second one, according to the position O
- ro,,s,h ,rys,,I ~ ,,yngr,, una Introd,ral AMI