to the axis o f the fcrew. The revolutions o f the fcrew are
counted on a fcale, which is an inch divided into fifty parts; the
index to thefe divifions is, a flower de luce marked upon the
Aider, which carries the needle point acrofs the field of the micro-
fcope. Every revolution o f the micrometer fcrew meafores
part o f an inch, which is again fubdivided by means o f the
divifions on the circular plate, as this is divided into twenty equal
parts, over which the index paffes at .every revolution o f the
fcrew; by which means, we obtain with eafe the meafure o f one
thoufandth part o f an inch : for 50, the number o f threads on the-
fcrew in one inch, being multiplied by 20 the divifions on the
circular plate, are equal to 1000 ; fo that each divifion on the circular
plate fhews, that the needle has either advanced or receded
one thoufandth part of an inch.
T o place this micrometer on the body o f the mierofeope, open;
the circular part F K H, Fig. 8, Plate II. A, by taking out the
fcrew G , throw back the femicircle F K which moves upon a
joint at K, then turn the Aiding tube o f the body of the micro-
fcope, fo that the fmall holes which are in both tubes may exaQJy
coincide, and let the needle g of the micrometer have a free
paffage through them; after this, fcrew it fall upon the body by
the fcrew G.
The needle will now traverfe the field of the mierofeope, and
meafure the length and breadth of the image of any ofejeft that is
applied to it. But further affiftance mult be had, in order to
meafure the objedt itfelf, which is a fubjeft of real importance;
for though we have afeertained the power o f the mierofeope, and
know that it is fo^many thoufand times, yet this will be bf little
^ affiftance
affiftance towards afeertaining an accurate idea o f it s real fize;
for our ideas o f bulk being formed by the comparifon of one ob-
j ea with another, we can only judge o f that of any particular
body, by comparing it with another whofe fize is known: the
fame thing is neceffary, in order to form an eftimate by the
mierofeope; therefore, to afeertain the real meafure o f the ob-
ieft, we muft make the point o f the needle pafs over the lipage of
a known part of an inch placed on the ftage, and write down the
revolutions made by the fcrew, while the needle paffed over the
image o f this known meafure; by which means, we afeertain the
number o f revolutions on the fcrew, which are adequate to a real
and known meafure on the ftage. As it requires an attentive eye
to watch the motion of the needle point, as it paffes over the
image of a known part of an inch on the ftage, we ought not to
truft to one Angle .meafurement of the image, but ought to repeat
it at leaft fix times; then add the fix meafures thus obtained together,
and divide their fum by fix, or the number of trials ; the
quotient will be the mean of all the trials. This reffilt is to be
placed in a column o f a table, next to that which contains the
number of the magnifiers.
By the affiftance o f the fe&oral fcale, we obtain with eafe a
fmall part of an inch. This fcale is ffiewn at Fig. 5, 6, 7, Plate
II. A, in which, the two lines ca cb, with the fide a b, form an
ifofceles triangle; each of the fides is two inches long, an t e
bafe one-tenth of an inch. The longer fides may be of any given
length, and the bale «ill only o f one-tenth o f an inch. 1 he
longer lines may b e -confidered as the line o f lines upon a fedor
„ i d t o o » « « * of an inch.Jpalpato S cb Hence, « t a » « m f m » ant divided to o . ™ fc e r f c naofnee »Jl