The pidture, formed by a convex lens, is either larger or lefs
than the object, in proportion as it’s diftance from tire lens is
greater or lefs than it’s diftance from the objedt.
As an objedt becomes perceptible to us, by means o f the image
thereof which is formed on the retina, it will, therefore, be feen
in that direction, in which the rays enter the eye to form the
image, and will always be found in the line, in which the axis o f
a pencil of rays flowing from it enter the eye. We from hence
acquire a habit o f judging the objedt to be fituated in that line.
Note ; as the mind is unacquainted with the refradtion the rays
fuffer before they enter the eye, it judges them to be in the line
produced back, in which the axis of a pencil o f rays flowing from
it is fituated, and not in that in which it was before the refradtion.
I f the rays, therefore, that proceed from an objedt, are refracted
and refledted feveral times before they enter the eye,
and thefe refradtions, or refledtions, change confiderably the
original diredtion o f the rays which proceed from the objeft ; it
is clear, that it will not be feen in that line, which would come
ftrait from it to the e y e ; but it will be feen in the direflion of
thofe rays which enter the eye, and form the image thereof on it.
We perceive the prefence and figure o f objedts, by the im-
preffion each refpedtive image makes on the retina ; the mind, in
confequence of thefe impreflions, forms conclufions concerning
the fize, pofition, and motion of the objedt. It muft however be
obferved, that thefe conclufions are often redtified or changed
by the mind, in confequence of the effedts o f more habitual impreflions.
For example; there is a certain diftance, at which,
in the general bufinefs o f life, we are accuftomed to fee objedts :
now, though the meafure o f the image o f thefe objedts changes
confiderably when they move from, or approach nearer to us, yet
we do not perceive that their fize is much altered: but beyond
this diftance, we find the objedts appear to be diminilhed, or in-
creafed, in proportion as they are more or lefs diftant from us.
For inftance, i f I place my eye lucceflively at two, at four,
and at fix feet from the fame perfon, the dimenfions of the image
on the retina will be nearly in the proportion of 1, o f i, of
p, and confequently they fhould appear to be diminifhed in
the fame proportion; but we do not perceive this diminution,
becaufe the mind has redtified thé impreflion received on the
retina. T o prove this, we need only confider, that if we fee
a perfon at 120 feet diftance, he will not appear fo ftrikingly
fmall, as i f the fame perfon fhould be viewed from the top o f a
tower, or other building, 120 feet high, a fituation to which we
had not been accuftomed.
From hence, alfo, it is clear, that when we place a glafs
between the objedt and the eye, which from it’s figure changes
the diredtion of the rays o f light from the objedt, this objedt
ought not to be judged as if it were placed at the ordinary
reach o f the fight, in which cafe we judge of it’s fize more by
habit than by the dimenfions of the images formed on the retina:
but it muft be eftimated by the fize o f the image in the eye,
or by the angle formed at the eye, by the two rays which
come from the extremity o f the objedt.
F If