be Fuch a part of one-tenth, as is expreffed by their divifions.
Thus, if it be divided into ten equal parts, this will divide the
inch into one hundred equal parts; the firft divilion next c will
be equal to one hundredth part o f an inch, becaufe it is the tenth
part o f one-tenth o f an inch. If thefe lines are divided into
twenty equal parts, the inch will be by that means divided into
two hundred equal parts. Laftly, i f ab ca are made three
inches long, and divided into one hundred equal parts, we obtain
with eafe the one thoufandth part. The fcale is reprefented as
folid at Fig. 6, but as perforated at Fig. 5 and 7 ; fo that the light
palfes through the aperture, when the fectoral part is placed on
the HaOve.
T o ufe this fcale, firft, fix the micrometer, Fig. 8, Plate II. A,
to the body of the microfcope; then fit the fedtoral fcale, Fig. 7,
In the ftage, and adjuft the microfcope to it’s proper focus or
diftance from the fcale, which is to be moved till the bale appears
in the middle o f the field o f view 5 then bring the needle point g,
Fig. 8, (by turning the fcrew L) to touch one of the lines c a ex-
aftly at the point anlwering to 20 on the fe floral fcale. The
index a o f the micrometer, Fig. 8, is to be fet to the firft divilion,
and that on the dial plate to 20, which is both the beginning and
end of it’s divifions: we are then prepared to find the magnifying
power o f every magnifier in the compound microfcope which we
are ufing.
Example; every thing being prepared agreeable to the foregoing
direftions, fuppofe you are defirous o f afcertaining the
magnifying power o f the lens marked No. 4 ; turn the micrometer
fcrew, until the point o f the needle has palled over the magnified
image
image of the tenth part of one inch ; then the divifion, where the
two indices remain, will Ihew how many revolutions, and parts
o f a revolution, the fcrew has made, while the needle point
traverfed the magnified image of the one-tenth o f an inch ; fuppofe
the refult to be twenty-fix revolutions* of the fcrew, and
fourteen parts o f another revolution* this is equal to- 26 multiplied
by 20, added to 14 ; that is, 534 thoufandth parts o f an
inch.
The twenty-fix divifions found' on th eftrait fcale o f the
micrometer, while the point of the needle palfed over the magnified
image o f one-tenth part o f an inch, were multiplied by 20*
becaufe the circular plate C D, Fig. 8, is divided into 20 equal
parts; this produced 520; then adding the fourteen parts of the-
next revolution, we obtain the 534- thoufandth parts o f an inch*
or 3 tenths and 34 hundredth parts o f another tenth, which*
is the meafure of the magnified image o f one-tenth o f an inch,
at the aperture of the eye glaffes, or at their foci. Now i f we
fuppofe the focus of the two eye glaffes to be one inch, the double
thereof is two inches*; or if we reckon in the thoufandth
part o f an inch, we have two thoufand parts for the diftance of
the eye from the needlepoint of the micrometer. Again, i f we
take the diftance of the image from the objeft at die ftage at fix
inches, or fixthoufandths* and add thereto two thoufand* double
the diftance o f the focus o f the eye glafs, we {hall, have eight
thoufand parts of an inch for the diftance o f the eye from the object;
and. as from the propofition, page 55, we gather that the
glaffes double the image, we muft double the number 534 found
upon the micrometer, which then makes 1068: then, by the
following analogy, we {hall obtain the number o f times the
microfcope'