New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1872-12-23

ICE AND SNOW cRYsTALs-vELocuY OF LIour IN TRANS
RARENT BODIES.
There s hardly more beautiful and instructive eK-
ample OF this play OF molecular force than that FUR
nished by the OF water. You have seen the ex-
Qulsite frondlke forms produced by the capitalization
of a film OF water on cold window-paue You have
also probably noticed the beautiful rosetes tied together
by the crystazng force during the descent OF SHOW
shower very calm day. The SLOPES ana SUBMITS or
the Alps are loaded N Winter with these blossoms o the
frost. They vary infinitely in detail OF beauty but he
same angular magnitude Il preserved throughout AN
noxble power binds spears and spiol@ TO the angle OF
60. The common ice OF our lakes IS also rned III us
depoSluon Dy the same angie. 1Vu IH each
in freezing water small crystals OF stellar Shapes euo
star consisting Of six rays, with this angle OF 60 De
vealed in ordinary III sunbeam, OF falling t4at ~
electric beam, have an instrument deca
10 unlock the frozen molecules without disturbing the
According to the emission theory the velocity Of light
the undulating theory, the reverse is the case. This
point has been subjected TO the test OF an experiment
Fizeau and decided I'm favor Of the unduatory theory.
Whenever the two theories have come into collision TINS
h. been the result.
REFRACTIoN oCCAsIoNED BY DIFFERENCE or DENSITY.
Consider a small portion of issuing from
point of light distant that the portion may be regarded
practically straight Moving vertically downwards,
and impngng horizontal surface of glass, the wave
would 90 through the glass without change Of direction
But the velocity ia glass less than the velocity in
air. the would be retarded in passing into the
denser medium. But suppose the wave before re ching
the glass TO Obque TO the surface that enc OF the
tarde the other portions they enter the glass being
retarded in succession This retardaon of the one END
OF the wave causes II 10 swing round ana Change us
front, that when the has fully entered the glass
its course is Obque to its original direction According
to the undulatOry theory, light is thus refracted'
This the Professor illustrated by holding long rod
in horizontal position, and asking the audience to
consider It as a wave OF light coming plumb downward
from star Surprising the platform to be transparent,
this might be expected to pass through 1t without
change OF direction He then held the rod at an angle
OF 5 degrees to the floor and supposng the rod wave,
showed clearly that If entering the material OF the
platform 1t encountered denser medium the lower end
rod would be somewhat retarded before the upper
end reached the surface of the platform This would
change the front of the wave and consequently its
direction. This change constituted the refracion OF
light aud evidently prop ortionate to the density OF
the transparent bodyl
The two elements OF rapdity Of propagation, both of
sound and light IN any substance Whatever are eas-
tcity and density, and the enormous velocity Of Hght
is atanable because the ether Is at the same time or
nntesmal density and of elasticity 1t sur
rounds the atoms OF all bodies, but seems 10 De 20
acted upon by them tnst s o Is AS vond
for the diminished velocity OF light In refracng
bodies Now IN virtue of the crystalline architecture
tails possesses different denstes in different drectlons
and the consequence that some of these media trans
init light with different veOcities. Now, refraction
depends wholly upon the change OF velocity being great
est where the density is greatest Hence IN many erys
tails have two different reractions, ray OF Light
being divided by such crystals into This effect is
called double refracton.
ORDINARY AND EXTRAORDINARY PoLARIzED RAYS.
In water example, there is nothing in the grouping
Of the molecules 10 interfere with the perfect homogen-
city OF the ether but when water orystazes TO Ice the
different In plate OF fee the elasticity OF the
ether in direction perpendicular TO the surface OF
freezing is different from what it IL parallel TO the
face of freeZng is, therefore, double refTactng
substance Double TefractiOn is displayed IN partlcu-
larry impressive manner b. Iceland SPAR which IS crys
al
density in two directions in tins crystal ~ very great
the separation of the two halves OF the beam being
therefore, particularly striking Upon the screen is row
projected an image OF our carbon points Introducing
the spar, permit the beam which builds the Image 10
pass through it; instantly you have the single image
divided into two others. Casting upon the screen an
image OF the aperture through Whlch the nsht ISSUES
from the electric lamp and introducing the spar, two
luminous disks instead Of one appear Immediately upon
The two beams into which the spar divides the single
inedentbeam do not behave alike. one OF them ooeys
this is called the ordinary ray The other does not OBEY
constant do the INCIDENT and refraeted rays
e IN the same plane s theTIGIs caed b 5oe
OF the same depth; looked at through the
LIQUID the cup that contains the more ACKNOWLEDGEMENTS
ing LIQUID Will appear shallower than the other. Place
piece OF Iceland spar dot OF INK the TWO dOts are
but one appears nearer thar the other The
EST dot belongs 60 most sronglyrefracted ray which
in this is the ordinary ray Turn the spar round
and the extraordinary image OF the spot rotaes rOund
the ordinary one. This elect is now exhibited on the
screen before you.
THE two images of the carbon points produced by the
divided rays first appeared They were then concen-
trated by lenses into two bright spots Of light When
the crystal was rotated one spot became center around
which the other one revolved.l
ORIGIN OF THE TERM POLARIZATION"
The double reraetion of Iceland spar first treated
in IA work published by Erasmus Bartholimus In 1862
The celebrated Huyghens sought to account for the
phenomenon on the principle Of the theory. and
he succeeded in doing He made highly important
observations the distinctive characters Of the two
beams transmitted by the spar. Newton, rejecting on
the observations OF Huyghens, came to the conclusion
that each Of the beams had two sides; and from the
analogy Of this twosdedness with the tweendedness
OF a magnet wherein consists its polarity, the two
We shall study this subject OF the polarization Of light
with great and profit by means OF crystal OF tour
mane And let us start with clear conception OF an
ordinary beam Of light 1t has been already explained
executed the line OF propagation In the case OF
ordinary light are TO figure the etherparucles as
vibrating in all directions, or azmuths as IL Is some
times expressed across this line NOW, IN plate
OF tOurmslne cut parallel 10 the axis OF the
crystal, the beam OF incident hgnt Is ulWaeu
into two, one vibrating parallel 10 the axis Vi the
crystal the other at right angles 10 me aka Inc group
ing Of the Tedee sHd Il the H Vibrations incident
file crystal 10 these two directions. One OF these beams
namely that whose vibrations are perpendicular to
the ais is quenched with exceeding rapidly by the
crystal. that after having passed through very
a. ybrations reduced TO a single plane. In this
dton it is what call beam OF plane poarzed night
PoLARIzED RAYS QUESCHED WHEN CROSSED AT RIGHT
ANGLES.
4 moments reflection will show, IL what has been
stated be correct, that, placing second plate OF
tourmane with it's parallel to the first, the light
will pass through BOTH but that If the axes be crossed
the light that passes through the one plate VIII be
ouenched by the other total iDtercepon OF the ght
6einr the consequence The experiment is now before
you the image OF plate Of tournaline is upon the
Of the crystal little deepened nothing more. BY
means OF endless screw now turn one OF the crys
tails gradually round as long as the Two plates are OD
ous 10 each other certain portion flight gets through
but they at right angles 10 each other the space
common to both is q space OF darkness,
FIJI