Encyclopaedia Britannica > Volume 15, NIC-PAR

228
A ppeav-
aiice of Ob¬
jects thro’
jVledia of
different
Ifornis.
An object
nluiited in
thebodzon
appears
above its
true place.
OPT
eye at F, because HB, the upper part of the vessel
Jill obstruct the ray EH *, but when it is filletl with
water to the height GH, the ray at ^ being refrac.
ted at the surface of the water into the line Jvt, the
eye at F shall see the object by means oi that.
^ In like manner, an object situated in the horizon
appears above its true place, on account of the ie-
fraction of the rays which proceed from it m their
passage through the atmosphere, lor, first, It the o
Let be situated beyond the limits of the atmosphere,
m entering it will be refracted towards the per-
pendicular j that is, towards a line drawn from the point
where they enter, to the centre of the earth, which it,
the centre of the atmosphere : and as they pass on, they
will be continually refracted the same way, because they
a " all along entering a denser part, the centre ot whose
convexity is still the same point •, upon which account
the line thev describe will he a curve bending ow
wards: and therefore none of the rays that come from
that object can enter an eye upon the surface of the
earth, except what enter the atmosphere higher than
they need to do if they could come in a right line
from the object: consequently the. object must appear
above its proper place. Secondly, If the object be
placed within the atmosphere, the case is still the same
for the rays which flow from it must continually entei
adenser medium whose centre « below the-eye , and
therefore being refracted towards the centre, that is
downwards as before, those which enter the eye must
necessarily proceed as from some point above the ob¬
ject j whence the object will appear above its proper
P^Hence it is, that the sun, moon, and stars, appear
above the horizon, when they are just below it j and
higher than they ought to do, vvhen they are above i .
Likewise distant hills, trees, &c. seem to be higher
than they arc. . ,1 , •
Besides, The lower these objects are in the horizon
the greater is the obliquity with which the rays which
flow from them enter the atmosphere, or pass from the
rarer into the denser parts of it •, and therefore they ap¬
pear to be the more elevated by refraction: on which
account the lower parts of them are apparently more
elevated than the rest. Tins makes their upper and
under parts seem nearer than they are j as is evident
from the sun and moon, which appear of an oval form
when they are in the horizon, their horizontal diame¬
ters appearing of the same length that they would do
jf the rays suffered no refraction, while then* veitical
ones are thus shortened.
ICo. . . •
in the lines oc, b d, parallel to their first directions. Appear-
Producc these lines back till they meet in c: ^Ob.
he the apparent place of the point R *, and it ^ evident ^
from the figure, that it must be nearer the eye than that different
point ; and because the same is true of all other pencils FormSt
flowing from the object A B, the whole will be seen in ' ^
the situation f g, nearer to the eye than the hne A^*
2. As the rays RK, RL would not have entered the
eye, but have passed by it m the * directions K r, L t,
had they not been refracted in passing through the me¬
dium, the object appears brighter. 3. Ihe rays A h%
B L will be refracted at h and ^ into the less conver¬
ging lines h k, i /, and at the other surface into k M,
/ M, parallel to A h and B 1 produced *, so that the ex¬
tremities of the object will appear in the lines M k, M l
produced, viz. in/and £, and under as arge an angle
fM e, as the angle A (/ B under which an eye at </
would have seen it had there been no medium inter¬
posed to refract the rays : and therefore it appears
larger to the eye GH, being seen through the in¬
terposed medium, than otherwise it would have done.
But it is here to be observed, that the nearer the point
c appears to the eye on account of the refraction of
the ravs RK, RL, the shorter is the image/#, because
it is terminated by the lines M/and Mg, upon which
account the object is made to appear less j and there¬
fore the apparent magnitude of an object is not much
augmented by being seen through a medium of tins
O J
form
Farther, it is apparent from the figure, that the ef¬
fect of a medium of this form depends wholly upon its
thickness j for the distance between the lines R r and
e r, and consequently the distance between the points c
and R, depends upon the length of the line K ° :
Again, The distance between the lines AM and/ M
depends on the length of the line h k; but both k »
and k h depend on the distance between the surfaces
CE and DF, and therefore the effect of this medium
depends upon its thickness.
Prop. HI.
An object seen through a convex lens, appears
larger, brighter, and more distant, than with
the naked eye.
15s
Prop. II.
154
An object
seen thro’
a plane me
dium ap-
pearsnearer
and bright
erthan seen
by the nak¬
ed eye.
Plate
CCCLXXXI.
An object seen through a medium terminated by
plane and parallel surfaces, appears nearer,
brighter, and larger, than with the naked eye.
7*
For instance, let AB (fig. 7.) be the object, CHEF
the medium, and GH the pupil of an eye, which is here
drawn large to prevent confusion m the figure.—And,
1. Let RK, RL, be two rays proceeding from the point
R and entering the denser medium at K and L j these
rays will here by refraction be made to diverge less, and
to proceed afterwards, suppose in the lines K u, L 6 ; at
a and b, where they pass out of the denser medium, they
will be as much refracted the contrary way, proceeding
To illustrate this, let AB (fig. 8.) he the object, CD Seen thro
the lens, and EF the eye. 1. From A and B, the extre-^ ^
mities of the object, draw the lines AYr, BXr, crossingpearsjar<;t
each other in the pupil of the eye*, the angle ArB com-brighter,
prehended between these lines, is the angle under which and more
the object would be seen with the naked eye. But by g>
the interposition of a lens of this form, whose property
it is to render converging rays more so, the rays A I
and BX will he made to cross each other before they
reach the pupil. There the eye at E will not perceive
the extremities of the object by means of these rays (tor
they will pass it without entering), hut by some others
which must fall without the points Y and X, or be¬
tween them *, hut if they fall between them, they will
be made to concur sooner than they themselves would
have done : and therefore, if the extremities ot the ob¬
ject could not he seen by them, it will much less he seen
by these. It remains therefore, that the rays which
will enter the eye from the points A and B after refrac¬
tion, must fall upon the lens without the points \ and