Hi The Mi cr of cope explained. 
let the n.eedle g of the micrometer have a free paffags. 
through them, and fcrew it faft upon the body by the. 
fcrew G, palling through the two ears F and H, and 
it will then appear as at X, fig. i. Place the fe&oral 
fcale, fig. 26, which is drawn on glafs, on the ftage, 
and adjuft the microfcope to its proper focus., or diftance 
from the fcale, which Aide on the ftage until the largeft 
extremity appear in the middle of the field of view, and, 
bring the needle point, by turning the head L, to touch 
one of the lines c a, exadtly at the point anfwering to 
20, the index at a fig. 28, being fet to the. firft divifion, 
and that on the dial-plate, which could not be reprefented 
in this figure being fet to No. 20, which is both the be¬ 
ginning and ending of its divifions. We fhall then be 
prepared to find the magnifying powers of every magni¬ 
fier in any compound microfcope whatfoever, if the above 
cautions are attended too. 
Example. 
Every thing being prepared as we have diredled, and 
any magnifier, fuppofe No. 4, in the firfi: table to be 
ufed, turn the micrometer fcrew until the point of ths 
needle has paid over the magnified image of the 30th 
part of one inch, then the divifions where the two indices 
remain, will (hew how many revolutions and parts of a 
revolution the fcrew has made, while the needle-point 
traverfed that magnified image of i-ioth of an inch, 
the refult was twenty-fix revolutions of the fcrew, and. 
fourteen parts of another revolution, which is equal to 
26 x 20 -f- 14* xz 534 thoufandth parts of an inch.- 
Hence we derive an eafy.way to count the divifions on 
the micrometer, thus finding upon the straight fcale 
twenty-fix divifions while the needle point pafied over 
the magnified image of i-ipth part of an inch, which 
being multiplied by 20, becaufe the circular-plate C D. 
Fig. 28, is divided into twenty equal parts produces 52c, 
v then 
/ , . t ^ 
