vex, and confequently the magnifying power very great, die
field of view is fo fmall, and it is fo difficult to adjufl with accuracy
their focal diflance, that it requires fome practice to render
the'tile thereof familiar; at the fame time, the fmallnefs o f the
aperture to thefe lenfes has been found injurious to the eyes o f
fome obfervers: notwithftandfog, however, thefe defefts, the
great magnifying power, as well as the diftinS vifion which is
obtained by the ufe o f a deep Angle lens, more than counter-
ballances every difficulty and difadvantage. It was with this inftrument
that Leeuwenhoek and Swammerdam, Lyonet and Ellis
examined the minima of nature, laid open fome o f her hidden
recedes, and by their example Simulated others to the fame
P-Uffuit.
The conftruaion o f the fmgle microfcope is fo Ample, that it
,s fefceptible o f but little improvement, and has therefore undergone
but few alterations ; and thefe have been chiefly confined to
the mode o f mounting it, or the additions to it’s apparatus. The
greateft improvement this inftrument has received, was made by
Dr. Lieberkuhn, about the year j 740; .it confifted in placing the
fmall lens in the center o f a highly-polifhed concave fpeculum o f
filver, by which means he was enabled to reffefl a ftrong light
upon the upper furface o f an objeft, and thus examine it with
great eafe and pleafure. Before this contrivance, it was almoft
impoflible to examine fmall opake objefls with any degree o f
exaftnefs and fatisfaflion; for the dark fide o f the objeft being
next the eye, and alfo overfhadowed by the proximity o f
the inftrument, it’s appearance was neceflarily obfcure and
indiftinft.
Dr.
IHHHi
Dr. Lieberkuhn adapted a microfcope to every object; they
confifted o f a fhort brafs tube, at the eye end o f which a concave
filver fpeculum was fixed, and in the center o f the fpeculum a
magnifying lens: the objeft was placed in the middle o f the
tube, and had*a fmall adjuftment to regulate it to the focus; at
the other end o f the tube there was a piano convex lens, to con-
denfe and render more uniform the light which was reflefted
from the mirror. But all this pains was not beftowed upon trifling
©bjefls his were generally the moft curious anatomical preparations,
a few o f which, with their microfcopes, are (I believe)
depofited in the Britifh Mufeum. It will be proper, in this place,
to give fome account of M. Leeuwenhoek’s microfcopes, which
were rendered famous throughout all Europe, on account o f the-
numerous difcoveries he had made with them, as well as from his
afterwards bequeathing a part o f them to the Royal Society.
The microfcopes he ufed were all fingle, and fitted up in a convenient
Ample manner; each o f them confifted o f a very fmall
double convex lens, let into a focket between two plates rivetted
together, and pierced with a fmall hole; the objefft was placed on
a filver point or needle, which, by means o f fcrews adapted for
that purpofe, might be turned about, raifed or deprefied at pleafure,
and thus be brought nearer to, or be removed farther from
the glafs, as the eye o f the obferver, the nature o f the object, and
the convenient examination of it’s parts required. M. Leeuwenhoek
fixed his objefts, i f they were folid, to the foregoing
point with glu e ; if they were fluid, he fitted them on a little
plate of talc, or exceeding thin blown glafs, which he afterwards,
glued to the needle, in the fame manner as his other objefts. The
glaffes were all exceeding clear, and of different magnifying powers,
which were proportioned to the nature o f the objedl, and
the