Encyclopaedia Britannica > Volume 15, NIC-PAR

Theory. q p y
I,aw of This acceleration of light in refraction is contr-rv
Refraction, to the opinion of those philosophers who maintain
^ that illumination is produced by the undulation of an
elastic medium. Euler attempts to prove, by mecha¬
nical laws, that tire velocities of the incident' and re¬
fracted light, are proportional to the sines of incidence
and retraction, while our principles make them in this
ratio inversely, .Boscovich proposed a line experiment
for deciding this question. The aberration of the fixed
stars arises from the combination of the motion of light
wththe motion of the telescope by which it is observ¬
ed. 1 heretore this aberration should be greater or less
when observed by means of a telescope filled with water
according as light moves slower or swifter through wa¬
ter than through air. Pie was mistaken in the manner
in which the conclusion should be drawn from the ob-
, servation made in the form prescribed by him: and the
experiment has not yet been made in a convincing man¬
ner 5 because no fluid has been found of sufficient trans¬
parency to admit of the necessary magnifying power. It
js an experiment of the greatest importance to optical
science.
I c s.
209
that there is any impact in tins case, or that the reflect-
mg impact should cease at a particular obliquity ? Rrdmeton.
. At must be acknowledged that it is a very curious 1 v
circumstance, that a body which is perfectly transpa-,, Ir5
rent should cease to he so at a certain obliquity ; thatRT-at a
-1- — certain obliquity; that a
a great obliquity should not hinder light from pissing
fi om a void into a piece of glass ; but that the same wholly re-
obliquity should prevent it fi-om passing from the glassflected by
into a void. The finest experiment for illustrating t|jetranspareni
fact is, to take two pieces of mirror-glass, not silvered,SU
and put them together with a piece of paper between
them, forming a narrow margin all round to keep them
apart. Plunge this apparatus into water. When it is
Held nearly parallel to the surface of the water, every
t.ung at the bottom of the vessel will he seen cleariy
through the glasses; but when they are turned so as to
be inclined about 50 degrees, they will intercept the
iglit as much as if they were plates of iron. It will be
proper to soak die paper in varnish, to prevent water
irom getting between the glasses.
What is called the brilliant cut in diamonds, is such Theb'rU-
Cor. 2. If the light be moving svifhin . . ?, dlSPOS!fT .of tiie posterior facets of the diamond, Hant cut ia
substance, and if its velocity (-estimated ;n ,r paTnt [!lat t,le made to fi'fl upon them so obliquelydiamonds
that o'f •its^iicidem^1 dTt ^ f^T to admit" oft^
Plate the figure of last propositLn the excess of thl son U1 Tr\^ ^ ^ Cal,ed the ^bion.
Ulxxvh. of EF above the square of FI ^ 1 •!? fi f1,llshasla Fin back, and the facets are dll on the
excess of the square of KF abn tl amC ^ont’ and 80 disposed as to refract the rays into suflici-
Therefore the squire of the T 0f I>L’ oblj(lulties» to be strongly reflected from the posterior
.hrru;mema,ior:rdM^r of th tuS ±rr ,D oubl£,s,are ,rde by-rtt;ng °"e dkmo"11
nendicular veloritv If the,, f yi ^ v i° t ie Pe^‘ rose fashion, and another similar one is put behind it,
- 1 1 • herefore the initial perpendi- with their plane surfaces joined. Or, more frequently,
the outside diamond has the anterior facets of the bril-
liant, and the inner has the form of the inner part of a
brilliant. If they he joined with very pure and strong¬
ly refracting varnish, little light is reflected from the
separating plane, and their brilliancy is very consider-
, , . T,T- , ....wc. perpendi¬
cular velocity IK be precisely equal to the specific
velocity, the light will just reach the farther side of
the attracting stratum, as at B, where its peipendicular
velocity will be completely extinguished, and its motion
will be in the direction BT. But it is here under the
w srSkTdT '°Td8 ti,° t kh> ”4 ^ zyP sd:t
a'nd it „i,| y
EB, and finally emerge back from the refracting stra¬
tum into the transparent substance in an angle RDA
equal to KEF.
It the direction of the light be still more oblique, so
that its perpendicular velocity is less than the specific
velocity, it will not reach the plane ST, but be re¬
flected as soon as it has penetrated so far that the spe¬
cific velocity of the part penetrated (estimated by the
compounding part of the area of forces) is equal to its
perpendicular velocity. Tims the ray/E will describe
the path LdD a penetrating to 6 d, so that the corre¬
sponding area of forces a ?> c e Is equal to the square of
J !ts perpendicular velocity.
Hie extreme brilliancy of dew drops and of jewels
had often excited the attention of philosophers, and it
a ways appeared a difficulty how light was reflected at
i tronj tb? posterior surface of transparent bodies. It
-i or ec oir Isaac Newton his strongest argument
against the usual theory of reflection, viz. that it was
produced by impact on solid elastic matter. He was
the hrst who took notice of the-total reflection in great
iquities, and very properly asked how it can be said
VOL. XV. Part I. *
from the flat side, and the effect of the posterior facets
is much diminished. But doublets might he construct¬
ed, by making the touching surfaces of a spherical
form (of which the curvature should have a due pro¬
portion to the size of the stone), that would produce
an effect nearly equal to that of the most perfect
brilliant. ri^
Cor. 3. Since the change made on the square of the Refraction
velocity of the incident light is a constant quantity, itdiminisllc>.
follows, that the refraction will diminish as the velocity T ^ "T
of the incident light increases. For if L i in fig. 7. t^incrcas-
be a constant quantity, and EL be increased, it is evi-es.
dent that the ratio of E t, or its equal EF, to EL will
he diminished, and the angle LEE, which constitutes
the refraction, will be diminished. The physical cause
of this is easily seen : When the velocity of the incident
light is increased, it employs less time in passing through
the refracting stratum or space between the planes ST
and KR, and is therefore less influenced by the refrac¬
ting forces. A similar effect would follow if the trans¬
parent body were moving with great velocity towards
the luminous body.
Some naturalists have accounted for the different re-
* D d frangibility