M 48 i c r o s c o p i c a l E s s a y s .
pencils which proceed from the points o and b, will be received
on the lens F G, and by it carried parallel to the e y e ; confe-
quently, the correfpondent points of the image O P will be
vifible; and thofe which are fituate farther out towards H and I,
will not be feen. This quantity o f the image Q P, or vifible area,
is called the field o f view.
Hence it appears, that if the image be large, a very fmall
part of it will be vifible; becaufe the pencils of rays will for the
moll part fall without the eye glafs F G. And it is likewife plain,
that a remedy which would caufe the pencils, which proceed
from the extremes O and B of the objeft, to arrive at the eye,
will render a greater part o f it vifible ; or, in other words, enlarge
the field o f view. This is effefied by the interpofition o f a broad
lens D E (Fig. 5,) o f a proper curvature, at a fmall diftance from
the focal image. For, by that means, the pencil D M, which
would otherwife have proceeded towards H, is refrafted to the
eye, as delineated in the figure, and the mind conceives from
thence the exiftence o f a radiant point at Q, from which the rays
Jaft proceeded. In like manner, and by a parity of reafon, the
other extreme o f the image is feen at P, and the intermediate
points are alfo rendered vifible. On thefe confiderations it is,
that compound microfcopes are ufually made to confift of
an objeft lens L N, by which the image is formed, enlarged,
and inverted; an amplifying lens D E, by which the field of view
is enlarged, and an eye glafs or lens, by means o f which the eye
is allowed to approach very near, and confequently to view the
image under a very great angle o f apparent magnitude. It is now
cuftomary to combine two or more lenfes together at the eye
glafs, in the manner o f Euftachio Divini and M. • Joblot; by
which
which means, the aberration o f light from the figure is in fome
meafure correfted, and the apparent field increafed.
O f the Solar Microscope.
In this inftrument, the image of the objeft is thrown upon a
fcreen in a darkened room. It may be confidered under two
diftinft heads : ift, the mirror and lens, which are intended to
reflect the light of the fun upon the objeft; and 2dly, that
part which conftitutes the. microfcope, or which produces the
magnified image o f the objeft, Fig. 10, Plate I. Let N O repre-
fent the fide o f a darkened chamber, G H a fmall convex lens,
fixed oppofite to a perforation in the fide N O, A B a plane
mirror, or looking-glafs, placed without the room to refleft the
folar rays on the lens CD, by which they are converged and concentrated
on the objeft fixed at E F.
2. The objeft being thus illuminated, the ray which proceeds
from E will be converged by the lens G H to a focus K, on the
fcreen L M ; and the ray which comes from F will be converged
to I, and the intermediate points will be delineated between I and
K ; thus forming a pifture, which will be as much larger than
the objeft, in proportion as the diftance o f the fcreen exceeds
that o f the image from the objeft.
From what has been faid, it appears plainly, the advantages we
gain by microfcopes are, derived, firft, from their magnifying
power, b ' which the eye is enabled to view more diftinftly
the parts o f minute objefts : fecondly, that by their afliftance,
more light is thrown into the pupil o f the eye, than is done with-
G - out