Encyclopaedia Britannica > Volume 15, NIC-PAR

Irheory. q p rp
pparent magnltii^e of tlie image upon the retina, we easily
!acl K,JU-dge 0 the. dlstance of objects, as often as we are other-
- —'i mse acquainted with their magnitude; but as often as
we are ignorant of the real magnitude of bodies, we can
never, from their apparent magnitude, form any judo-e-
ment of their distance. 6
. Hence we may see why we are so frequently deceived
in our estimates of distance, by any extraordinary mag-
mtudes of objects seen at the end of it; as, in travelling;
towards a large city, or a castle, or a cathedral church
or a mountain larger than common, we fancy them to
be nearer than they really are. This also is the reason
why animals, and little objects, seen in valleys, conti¬
guous to large mountains, appear exceedinglv small,
for we think the mountain nearer to us than if it were
smaller ; and we should not be surprised at the small¬
ness of the neighbouring animals, if we thought them
farther off. For the same reason, we think them ex¬
ceedingly small, when they are placed upon the top of
a mountain, or a large building ; which appear nearer
to us than they really are, on account of their extraor¬
dinary size.
useen Dr Jurin accounts for our imagining objects, when
• n a high seen from a high building, to be smaller than they are,
1 j,jins and smaller than we fancy them to be when we view
.• ear them at the same distance on level ground. It is, says
t nthe kf> because we have no distinct idea of distance in that
E U 1Cy direction, and therefore judge of things by their pictures
upon the eye only ; but custom will enable us to judge
rightly even in this case.
Let a boy, says he, who has never been upon any
high building, go to the top of a lofty spire, and
look down into the street ; the objects seen there, as
men and horses, will appear so small as greatly to sur¬
prise him. But io or 20 years after, if in the mean
time he has used himself now and then to look down
horn that and other great heights, he will no longer
find the same objects to appear so small. And if he
were to view the same objects from such heights as fre¬
quently as he sees them upon the same level with
himself in the streets, he supposes that they would ap¬
pear to him just of the same magnitude from the
top of the spire, as they do from a window one story
high. For this reason it is, that statues placed upon
■very high buildings ought to be made of a larger size
than those which are seen at a nearer distance ; because
all persons, except architects, are apt to imagine the
height of such buildings to be much less than it really
I c s.
243
sent to our mind one object alone, but at the same 4 .
ciDalobiect86 tf!at Hr PlaCed betwixt US and the Prln- P'S&c.
^ I • 'j!10se ^lstance we are considering : and of objeets.
he more tins distance xs divided into separate and di-
reason dv’ ^ 14 appearS to be' For this
reason, d stances upon uneven surfaces appear less than
aS f°r 116 |ncfJualitJes of the surfaces, such
sJ ’;?,nd h?Ies’ and rivers> that lie low and out of
lieg behindht|r ^°r hinder tl,e Parts that
Daren? disf 7 appearf SJ so the whole ap-
appear n itnCeTb by tbe parts that da
PP ax in it. I his is the reason that the banks of a
1S9
iy ob-
ts seen
irwaaT™:rus to ad;sta"t cye’ *he --
<r f' Citrjssfn.-fS.t
of trees j for they seen, ,0 converge more and more "
they are farther extended from the eye. The reason
o t ns, xe says, is because the ap]iarent magnitudes of
their perpendicular intervals are perpetually diminish¬
ing, while, at the same time, we mistake their distance.
ence we may see why, when two parallel rows of
trees stand upon an ascent, whereby the more remote
parts appear farther off than they really are, because
the line that measures the length of the vistas now ap¬
pears under a greater angle than when it was horizontal
the trees, in such a case, will seem to converge less’
and sometimes, instead of converging, they will be
thought to diverge.
For the same reason that
The fourth method by which Dr Porterfield says
that we judge of the distance of objects, is the force
with which their colour strikes upon our eyes. For if
we be assured that two objects are of a similar and like
colour, and that one appears more bright and lively
than the other, we judge that the brighter object is the
nearer of the two.
Tbe fifth method consists in the different appearance
of the small parts of objects. When these parts appear
distinct, we judge that the object is near j but when
they appear confused, or when they do not appear at
all, we reckon the object to be at a greater distance.
For the image of any object, or part of an object, dimi¬
nishes as its distance increases.
The sixth and last method by which we judge of
the distance of objects is, that the eye does not repre-
a long vista appears to
converge more and more the farther it is extended
trom the eye, the remoter parts of a horizontal walk
or a long floor will appear to ascend gradually: and
objects placed upon it, the more remote they are the
higher they will appear, till the last be seen on a level
with the eye ; whereas the ceiling of a long gallery
appears to descend towards a horizontal line, drawn
from the eye of the spectator. For this reason, also,
the surface of the sea, seen from an eminence, seems
to rise higher and higher the farther we look} and the
upper parts of high buildings seem to stoop, or incline
forwards over the eye below, because they seem to ap¬
proach towards a vertical line proceeding from the
spectator’s eye 5 so that statues on the top of such build¬
ings, in order to appear upright, must recline, or bend
backwards.
Dr Porterfield also show's the reason why a windmill,
seen from a great distance, is sometimes imagined to
move the contrary way from what it really does, by
our taking the nearer end of the sail for the more’ re¬
mote. The uncertainty we sometimes find in the
course of the motion of a branch of lighted candles,
turned round at a distance, is owing, he says, to the
same cause j as also our sometimes mistaking a convex
for a concave surface, more especially m viewing seals
and impressions with a convex glass or a double mi¬
croscope } and lastly, that, upon coming in a dark
night into a street, in which there is but one row
of lamps, we often mistake the side of the street they
are on.
Far more light was thrown upon this curious subject
by M. Bouguer.
The proper method of drawing the appearance of
H h 2 two