Encyclopaedia Britannica > Volume 15, NIC-PAR

OPT
Plate
ccclxxxiii.
fig.
25 2
On the dif-yellow, and orange, in order. The same thing happen-
ferent re- ed when I removed the prism out of the sun’s light, an
frangibility ]00ked through it upon the hole shining by the light ot
,of U£hi , the clouds beyond it.. And yet if the refractions of all
* ' the rays were equal according to one certain proportion
of the sines of incidence and refraction, as is vulgarly
supposed, the refracted image ought to have appeared
round, by the mathematical demonstration above men¬
tioned. So then by these two experiments it appears,
that in equal incidences there is a considerable inequa¬
lity of refractions.”
For the discovery of this fundamental property 01
light, which has unfolded the whole mystery of colours,
we see our author was not only beholden to the experi¬
ments themselves, which many others had made before
him, but also to his skill in geometry; which was abso¬
lutely necessary to determine what the figure of the re¬
fracted image ought to be upon the old principle of an
equal refraction of all the rays : but having thus made
the discovery, he contrived the following experiment to
prove it at sight. _ _ T
“ In the middle of two thin boards, Ilk1
made a round hole in each, at G and g, a third part of
an inch in diameter 5 and in the window-shut a much
larger hole being made, at F, to let into my darkened
chamber a large beam of the sun’s light, I placed a
prism, ABC, behind the shut in that beam, to refract
it towards the opposite wall and close behind this prism
1 fixed one of the boards DE, in such a manner tnat the
middle of the refracted light might pass through the
hole made in it at G, and the rest be intercepted by the
board. Then at the distance of about 12 feet from the
first board, I fixed the other board, de, in such manner
that the middle of the refracted light, which came
through the hole in the first board, and fell upon the
opposite wall, might pass through the hole g in this
other board d e, and the rest being intereepted by the
board, might paint upon it the coloured spectrum of the
sun. And close behind this board I fixed another prism
a be, to refract the light which came through the hole
g. Then I returned speedily to the first prism ABC,
and by turning it slowly to and fro' about its axis, I
caused the image which fell upon the second board d e,
to move up and down upon that board, that all its parts
might pass successively through the hole on that board,
and fall upon the prism behind it. And in the mean
time I noted the places, M, N, on the opposite wall, to
which that light after its refraction in the second prism
did pass j and by the difference of the places at M and
N, I found that the light, which being most refracted
in the first prism ABC, did go to the blue end of the
image, was again more refracted by the second prism
a b c, than the light which went to the red end of that
image. For when the lower part of the light which
fell upon the second board de, was cast through the
hole g, it went to a lower place M on the wall •, and
when the higher part of that light was cast through the
same hole g, it went to a higher place N on the wall •,
and when any intermediate part of the light was cast
through that hole, it went to some place in the wall be¬
tween M and N. The unchanged position of the hole
in the boards made the incidence of the rays upon the
second prism to be the same in all cases. And yet in
that common incidence some of the rays were more re¬
fracted and others less ; and those were more refracted
1 c s. Part L
in this prism, which by a greater refraction in the first On the dif-
prism were more turned out of their way j and, there- ^rent«.
fore, for their constancy of being more refracted, are of
deservedly called more refrangible” »
Sir Isaac shows also, by experiments made with con- 20s
vex glass, that lights, reflected from natural bodies Refected
which differ in colour, differ also in refrangibility i a»(l3yd,?fr'
that they differ in the same manner as the rays of the frangiyei
snn do. „ n m •
44 The sun’s light consists of rays filtering in retiexibi-
lity, and those rays are more reflexible than others which
arc more refrangible. A prism, ABC, whose two Iig. 4,
angles, at its base BC, were equal to one another and
half right ones, and the third at A a right one, 1 pla¬
ced in a beam FM oi the sun’s light, let into a dark,
chamber through a hole F one third part of an inch
broad. And turning the prism slowly about its axis un¬
til the light which went through one of its angles ACB,
and was refracted by it to G and H, began to be re¬
flected into the line MN by its base BC, at which till
then it went out of the glass ; I observed that those rays,
as MH, which had suffered the greatest refraction, were
sooner reflected than tl>e rest. lo make it evident that
the rays which vanished at H were reflected into the
beam MN, I made this beam pass through another
prism VXY, and being refracted by it to fall afterwards
upon a sheet of white paper p t placed at some distance
behind it, and there by that refraction to paint the usual
colours at p t. Then causing the first prism to be turned
about its axis according to the order of the letters ABC,
I observed, that when those rays MH, which in this
prism had suffered the greatest refraction, and appeared
blue and violet, began to be totally reflected, the blue
and violet light on the paper which was most refracted
in the second prism received a sensible increase at p,
above that of the red and yellow at t: and afterwards,
when the rest of the light, which was green, yellow,
and red, began to be totally reflected and vanished at
G, the light of those colours at t, on the paper p t, re¬
ceived as great an increase as the violet and blue had
received before. Which puts it past dispute, that those
rays became first of all totally reflected at the base BC,
which before at equal incidences with the rest upon the
base BC had suffered the greatest refraction. I do not
here take any notice of any refractions made in the
sides AC, AB, of the first prism, because the light en¬
ters almost perpendicularly at the first side, and goes
out almost perpendicularly at the second •, and therefore
suffers none, or so little, that the angles of incidence
at the base BC are not sensibly altered by it; especially
if the angles of the prism at the base BC be each about
^.0 degrees. For the rays FM begin to be totally re¬
flected when the angle CMF is about 50 degrees, and
therefore they will then make a right angle of 90 de¬
grees with AC.
“It appears also from experiments, that the beam
of light MN, reflected by the base of the prism, being
augmented first by the more refrangible rays ami after-
wards by the less refrangible, is composed ot rays dii-
ferently refrangible. .
“ The light whose rays are all alike refrangible, 1 call
simple, homogeneous, and similar ; and that whose rays
are some more refrangible than others, I call compound,
heterogeneous, and dissimilar. The former light 1 call
homogeneous, not because I would affirm it so in all re¬
spects 5