microfcope magnifies the diameter o f the objeft; fay, as 240,
the diftance o f the eye from the image o f the object, is to 800,
the diftance o f the eye from the object, fo is 1068, double the
ineafure found on the micrometer, to 3563, or die number o f
times the microfcope magnifies the diameter o f the objeft. By
working in this manner, the magnifying power o f each lei* tiled
with die compound microfcope, may be eafily found, though the
refult will be different in different compound microfcopes, varying
according to the combination of the lenfes, their diftance
from the objeft and one another, &c.
Having difcovered the magnifying power of the microfcope,
with the different objeft lenfes that are ufed therewith, our next
fubjeft is to find out the real fize o f the objects themfelves, and
their different parts ; this is eafily effefted, by finding how many
revolutions o f the micrometer-fcrew anfwer to a known meafure
on the feftoral fcale, or other objeft placed on the ftage; from
the number thus found, a table fhould be conftrufted, exprefling
the value of the different revolutions o f the micrometer with that
objeft lens, by which the primary number was obtained. Similar
tables muff be conftrufted for each objeft lens. By a fet o f tables
o f this kind, the obferver may readily find the meafure of any
objeft he is examining; for he has only to make the needle point
traverfe over this objeft, and obferve the number of revolutions
the fcrew has made in it’s paffage, and then look into his table
for the real meafure which correfponds to this number o f revolutions,
which is the meafure required.
C H A P .
C H A P . I I I .
A Description of the most approved Microscopes,
and the Method of using them.
IN the preceding chapter I have endeavoured to give a com-
prehenfive view of the theory of the microfcope, and the
principles on which the wonderful effefts o f this infiniment depend.
I ftiall now proceed to dèfcribe the various inftruments
themfelves, their apparatus, and the moft eâfy and ready mode of
applying them to ule ; felefting for defcription thofe, that from
fome peculiar advantage in their conftruftion, or from the reputation
o f the authors who have recommended and ufed them, are
in moft general ufe. What is faid of the le will, I hope, be
fufficient to enable the reader to manage any other kind that may
fall in his way.
Description of A dams’s improved and universal
L ucernal Microscope, Fig. 1, Plate III.
This microfcope was originally thought of, and in part executed
by my father ; I have, however, fo improved and altered
it, both in conftruftion and form, as to render it altogether a
different inftrument. The approbation it has received from the
moft experienced microfcopic obfervers, as well as the great
X demand