to A B ; that is, i f we would fee the objeft under an angle as
large as F E G, or would make it appear the fame length that an
objeft as long as F G would appear, it maybe done by coming
nearer to the objeft. For the apparent diameter is as the diftance
inverfely; therefore, i f C D is as much lefs than C E, as F G is
greater than A B , by bringing the eye nearer to the objeft in the
proportion o f C D to E D , the apparent diameter will be magnified
in the proportion o f F G to A B ; fo that the objeft A B ,
to the eye at D, will appear as long as an objeft F G would appear
to the eye at E. In the fame manner, we might {hew, that
the apparent diameter-of an objeft, when feen by the naked eye,
may be infinite. For fince the apparent diameter is reciprocally
as the diflance of the eye, when thé diftance o f the eye is nothing,
or when the eye is clofe to the objeft at C, the apparent diameter
will be the reciprocal o f nothing, or infinite.
There is, however, one great inconvenience in thus magnifying
an objeft, without the help o f glaffes, by placing the eye
nearer to it. The inconvenience is, that we cannot fee an objeft
diftinftly, unlefs the eye is about five or fix inches from i t ; therefore,
i f webring it nearer to our eye than five or fix inches, however
it may be magnified, it will be feen confufedly. Upon this
account, the greateft apparent magnitude o f an objeft that we
are ufed to, is the apparent magnitude, when the eye is about five
or fix inches from it: and we never place an objeft much within
that diftance; becaufe, though it might be magnified by this
meansy -yet the confufion would prevent our deriving any advantage
from feeing it fo large. The fize o f an objeft feems extraordinary,
when viewed through a convex lens ; not becaufe it is
impoflible to make it appear o f the fame fize to the naked eye,
4 but
but becaufe at the diftance from the eye which would be neceffary
for this purpofe, it would appear exceedingly confufed; for
which reafon, we never bring our eye fo near to it, and confe-
quently, as we have not been accuftomed to fee the objeft of this
fize, it appears an extraordinary one.
On account o f the extreme minutenefs of the atoms of light,
it is clear, a Angle ray, or even a fmall number o f rays, cannot
make a fenfible impreflion on the organ of fight, whofe fibres are
very grofs, when compared to thefe atoms; it is neceffary, therefore,
that a great number Ihould proceed from the furface o f an
objeft, to render it vifible. But as the rays of light, which
proceed from an objeft, are continually diverging, different
methods have been contrived, either of uniting them in a given
point, or of feparating them at pleafure : the manner of doing
this, is the fubjeft of dioptrics and catoptrics.
- By the help o f glaffes*, we unite in the fame fenfible point a
great number of rays, proceeding from one point of an objeft;
and as each ray carries with it the image of the point from whence
it proceeded, all the rays united mull form an image o f the
objeft from whence they were emitted. This image is brighter,
in proportion as there are more rays united ; and more diftinft,
in proportion as the order, in which they proceeded, is better
preferved in their union. This- may be rendered evident; for, if
a white and polifhed plane is placed where the union is formed,
we fhall fee the image of the objeft .painted in all it’s colours on
this plane; which image will be brighter, if all adventitious light
is excluded from the plane on which it is received.