limits o f diftinft vifion with the naked eye) to eight inches. For
example, i f the femi-diameter o f a lens, equally convex on both
6des, be half an inch, which is alfo equal to it’s focal diftance,
we {hall have as 4 is to 8, fo is l to 16 : that is, the diameter
o f the objeft in the proportion o f fixteen to one. 2. As the
diftance o f eight inches is always the fame, it follows, that by
how much the focal diftance is fmaller, there will be a greater
difference between it and the eight inches; and confequently,
the diameter o f the objeft will be fo much the more magnified,
in proportion as the lenfes are fegments o f fmaller fpheres. 3. I f
the objeft be placed in the focus o f a glafs globule or fphere, and
the eye be behind it in the focus, the objeft will be feen diftinft
in an ereft fituation, and magnified as to it’s diameter, in the proportion
o f 4 of the diameter o f the globule to eight inches;
thus fuppofe the diameter o f the fphere to be A o f an inch,
then j of this will be equal to 4V; confequently, the real
diameter o f the objeft to the apparent one, as A to 8, or as
3 to 320, or as 1 to 160 nearly.
O f the D ouble or Compound Microscope.
In the compound microfcope, the image Is viewed inftead o f
the objeft, which image is magnified by a {ingle lens, as the objeft
is in a fingle microfcope. It co nftfts o f an objeft lens, L N,
Fig. 5, Plate I. and an eye glafs F G . The objeft O B is placed
a little further from the lens than it’s principal focal diftance, fo
that the pencils o f rays proceeding from the different points of
the objeft through the lens, may converge to their refpeftive
foci, and form an inverted image o f the objeft at P O ; which
image is viewed by the eye through the eye glafs F G, which is
fo
fo placed, that the image may be in- it’s focus on one fide, and
the eye at the fame diftance on the other. The rays o f each pencil
will be parallel, after paffing out o f the glafs, till they reach
the eye at E, where they will begin to converge by the refraftive
powers of the humours; and after having crofted each other
in the pupil, and pafled through the chryftalline and vitreous
humours, they will be collefted in points on the retina, and form
a large inverted -image thereon.
It will be eafy, from what has been already explained, to underhand
the reafon o f the magnifying power o f a compound
microfcope. The objeft is magnified upon two accounts; firft,
becaufe if we viewed the image with the naked eye, it would
appear as much larger than the objeft, as the image is really
larger than it, or as the diftance f R is greater than the diftance
f b ; and fecondly, becaufe this pifture is again magnified by the
eye glafs, upon the principle explained in the foregoing article
on vifion, by fingle microfcopes.
But it is to be noted, that the image formed in the focus o f a
lens, as is the cafe in the compound microfcope, differs from the
real objeft in a very eflential particular; that is to fay, the light
being emitted from the objeft in every direction, renders it vifible
to an eye placed in any pofition; but the points o f the image
formed by a lens, emitting no more than a fmall conical body o f
rays, which, arrives from the glafs, can be vifible only when the
eye is fituate within it’s confine. Thus the pencil, which
emanates from o in the objeft, and is converged by the lens to
M, proceeds afterwards diverging towards H, and therefore,
never arrives at the lens F G, nor enters the eye at E. But die
pencils