OPT 
’now many revolutions, and -parts of a revolution, the 
fcrew has made, while the needle-point traverfed the. 
magnified image of the one-tenth of an inch. Suppofe 
the refult to be 26 revolutions of the fcrew, and 14. parts 
of another revolution : this is equal to 26 multiplied by 
20, added to 14.; that is, 534,000 parts of an inch. 
The 26 divifions found on the ftraight fcale of the micro¬ 
meter, while the point of the needle palled over the mag¬ 
nified image of one-tenth part of an inch, were multi¬ 
plied by 20, becaufe the circular plate CD, fig. 12, is di¬ 
vided into 20 equal parts; this produced 520; then ad¬ 
ding the parts of the next revolution, we obtain the 
534,000 parts of an inch, or five-tenths and 3400 parts of 
another tenth, which is the meafure of the magnified 
image of one-tenth of an inch, at the aperture of theeye- 
glafles or at their foci. Now, if 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 1000th part of an 
inch, we have 2000 parts for the diftance of the eye from 
the needle-point of the micrometer. Again, if we take 
the diftance of the image from the objedt at the ftage at 6 
inches, or 6000, and add thereto 2000, double the diftance 
of the focus of the eye-glafs, we {hall have 8000 parts of 
an inch for the diftance of the eye from the object; and, 
as the glaifes double the image, we mull double the num¬ 
ber 534 found upon the micrometer, which then makes 
3068: then, by the following analogy, we Ural! obtain 
the number of times the microfcope magnifies the diame¬ 
ter of the objedt ; fay, As 240, the diftance of the eye 
from the image of the objedt, is to 800, the diftance of 
the eye from the objedt; fo is 1068, double the meafure 
found on the micrometer, to 3563, or the number of 
times the microfcope magnifies the diameter of the 
objedt. By working in this manner, the magnifying 
power of each lens ufed with the compound microfcope 
may be eafily found, though the refult will be different in 
different compound microlcopes, varying according to the 
combination of the lenfes, their diftance from the objedt 
and one another, &c. 
Having difcovered the magnifying-power of the micro¬ 
fcope, with the different objedl-lenfes that are ufed there¬ 
with, our next fubjedf is to find out the real fize of the 
objedts themfelves, and their different parts: this is eafily 
effedted, by finding how many revolutions of the micro¬ 
meter-fcrew an fwer to a known meafure on the fedloral 
fcale or other objedt placed on the ftage; from the num¬ 
ber thus found, a table fttould be conftrudted, expreffing 
the value of the different revolutions of the micrometer 
with that objedt-lens by which the primary number was 
obtained. Similar tables mull be conftrudted for each 
objedt-lens. By a fet of tables of this kind, the obfer- 
ver may readily find the meafure of any objedt he is exa¬ 
mining ; for he has only to make the needle-point tra- 
verfe over this objedt, and oblerve the number of revolu¬ 
tions the fcrew has made in its paffage, and then look 
into his table for the real meafure which correfponds to 
this number of revolutions, which is the meafure re¬ 
quired. 
Cavallo’s mother-of-pearl micrometer, before defcribed, 
may equally be applied to a microfcope; and it will thus 
ferve to meafure the lineal dimenfions of the objedt; and 
the value of its divifions are afcertained by placing an 
objedt of a known dimenfion before the microfcope, and 
by obferving how many divifions of the micrometer mea¬ 
fure its magnified image: for inftance, place a piece of 
paper, which is exadtly one-tenth of an inch long, before 
the microfcope ; and, if you find that 50 divifions of the 
micrometer meafure its magnified image, you may con¬ 
clude that each divifion is equal to, or rather denotes an 
extenfion of, the 500th part of an inch, in the objedt ; 
for, if 50 diyifions meafure one-tenth, 500 divifions mull 
meafure the whole inch ; &c. 
But the molt minute and accurate micrometers appear 
to be thofe invented by Mr. Coven try of South wark, (whofe 
curious hygrometer we have mentioned in vol. x.) and 
I C S. 631 
defcribed in the Encyclopaedia Britannisa, to the editor 
of which fome of them were fent for that purpofe. 
“ This micrometer may be compofed of glafs, ivory, 
or filver, on which are ufually drawn parallel lines from 
the 10th to the 10,oooth part of .an inch. But one of 
thofe which Mr. Coventry has fent 11s is divided into 
fquares, fo fmall that fixteen millions of them are con¬ 
tained on the furface of one fquare inch, each fquare ap¬ 
pearing under the microfcope true and diftindt; and, 
though fo fmall, it is a fadt, that animalcula are found 
which may be contained in one of thefe fquares.” 
The life of micrometers, when applied to microfcopes, 
is to meafure the natural fize of the objedt, and how 
much that objedt is magnified. To afcertain the real 
fize of an objedt in the fingle microfcope, nothing more 
is required than to lay it on the micrometer, and adjuft 
it to the focus of the magnifier, noticing how many divi¬ 
fions of the micrometer it covers. Suppofe the parallel 
lines of the micrometer to be the 1000th of an inch, and 
the objedt covers two divifions; its real fize is the 500th 
part of an inch; if five, the 200th part; and fo on. 
In ufing the compound microfcope, the real fize of the 
objedt is found by the fame method as in the fingle; but, 
to demonftrate the magnifying-power of each glafs to 
greater certainty, adopt the following method : On the 
ftage in the focus of the object-glafs lay a micrometer, 
coniifting of an inch divided into 100 equal parts ; count 
how many divifions of the micrometer are taken into the 
field of view; then lay a two-foot rule parallel to the 
micrometer: fix one eye on the edge of the field of light, 
and the other eye on the end of the rule, which move, 
till the edge of the field of light and the end of the rule 
correfpond; then the diftance from the end of the rule 
to the middle of the ftage will be the half of the diameter 
of the field : e. gr. if the diftance be ten inches, the whole 
of the diameter will be twenty, and the number of the 
divifions of the micrometer contained in the diameter of 
the field is the magnifying-power of the microfcope. Thus, 
fuppofe the number of divifions feen in the micrometer 
to be-j^ths, or \ an inch, and that the diameter of the 
field, meafures with the rule 20 inches; the ■££0 01 ' i of 
an inch is only the 40th part of 20 inches ; it Ihows "the 
magnifying-power of the glafs to be 40 times in length ; 
40X40, or 1600 times, the fuperficies; and 1600X40 = 
64,000, the magnified cube of the objedt. In like man¬ 
ner each objedt-glafs, or magnifier, muft be proved ; and 
a table kept of their feyeral magnifying powers. 
Another way of finding the magnifying-power of com¬ 
pound microfcopes, is by ufing two micrometers of the 
fame divifions ; one adjufted under the magnifier, the 
other fixed in the body of the microfcope in the focus of 
the eye-glafs. Notice how many divifions of the micro¬ 
meter in the body are feen in one divifion of the micro¬ 
meter under the magnifier, which again muft be multi¬ 
plied by the power of the eye-glafs. Example: Ten di- 
vifions of the micrometer in the body are contained in 
one divifion under the magnifier; fo far the power is in- 
creafed ten times : now, if the eye-glafs be one inch fo¬ 
cus, fuch glafs will of itfelf magnify about feven times in 
length, wftiich, with the ten times magnified before, 
will be 7 X 10 — 70 times in length, 4900 fuperficies, 
and 343,000 cube. 
“If (fays Mr. Coventry) thefe micrometers are em¬ 
ployed in the folar microfcope, they divide the objedt into 
fquares on the fcreen in fuch a manner as to render it ex¬ 
tremely eafy to make a drawing of it. And I apprehend 
they may be employed to great advantage with fuch a 
microfcope as Mr. Adams’s lucernal; becaufe this inftru- 
ment may be ufed either by day or night, or in any place, 
and gives the adtual magnifying-power without calcula¬ 
tion.” 
The cafe with which we have been favoured by Mr. 
Coventry (fay the editors of the Ency. Brit.) contains 
fix micrometers, two on ivory and four on glafs. One-of 
thofe on ivory is an inch divided into one hundred parts, 
1 every 
