Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

jDefrription
of Mr l o-
ventry’s mi¬
crometers
for micro-
feopes.
MIC [806
be different in different compound microfcopes, varying
according to the combination of the lenles, their diltance
from the object and one another, &c.
61. Having dilcovered the magnifying power of the
microfcope, with the different objett-lenies that are uled
therewith, our next fub e£t is to find out the real iize
of the objects themfelves, and their different parts :
this is eafily effedted, by finding how many revolutions
of the micrometer ferew anfwer to a known meafure
on the lecloral Icale or other object placed on the
ftage *, from the number thus found, a table fhould be
conltrudled, expreffing the value of the different revo¬
lutions of the micrometer with that ob odt lens, by
which the primary number was obtained. Similar
tables muff be conffrudted for each objedi lens. By a
fet of tables of this kind, the obferyer may readily find
the meafure of any objedt he is examining •, for he has
only to make the needle point traverfe over this objedt,
and obferve the number of revolutions the ferew has
made in its paffage, and then look into his table for the
real meafure which correfponds to this number of revo¬
lutions, which is the meature required.
62. Mr Coventry of Southwark has favoured us with
the defeription of a micrometer of his own invention j
the feale of which, for minutenefs, finpaffes every initru-
raent of the kind of which we have any knowledge, and
of which, indeed, we could fcarcely have formed a con¬
ception, had he not indulged us with fevcral of thefe
inftruments, graduated as underneath.
The micrometer is compofed of glafs, ivory, filver,
&c. on which are drawn parallel lines from the 10th
to the 1 o.oooth part of an inch. But an inftrument
thus divided, he obferve^, is more for curiofity than
ufe: but one of thofe which Mr Coventry has lent us
is divided into fquares, fo fmall that fixteen millions of
them are contained on the furface of one fquare inch,
each fquare appearing under the microfcope true and
diftinft j and though fo fmall, it is a fail, that animal-
cula are found which may be contained in one of thefe
fquares.
The ufe of micrometers, when applied to micro¬
fcopes, is to meafure the natural fize of the objedt,
and how much that objedl is magnified. To afeertain
the real fize of an objedl in the fingle microfcope, no¬
thing more is required than to lav it on the microme¬
ter, and adjuft it to the focus of the magnifier, no¬
ticing how many divifions of the micrometer it covers.
Suppofe the parallel lines of the micrometer to be the
1000th of an inch, and the objedl covers two divifions j
its real fize is 500ths of an inch j if five, 20Cths, and
fo on.
But to find how much the objedl is magnified,
is not mathematically determined fo eafily by the fingle
as by the compound microfcope : but the follow¬
ing fimple method (fays Mr Coventry) I have ge¬
nerally adopted, and think it tolerably accurate.
Adjuft a micrometer under the microfcope 0, fay the
t ooth of an inch of divifions, with a fmall objedl on
it •, if fquare, the better : notice how many divifions one
fide of the objedt covers, fuppofe ro : then cut a piece
] MIG
of white paper fomething larger than the magnified ap- Microtae.
pearance oi the objedt : then fix one eye on the objedt ter>
through the microlcope, and the other at the fame time v~—
on the paper, lowering it down til! the objedi and the
paper appear level and diiiindt : then cut the paper till
it appear exadtly the fize of the magnified objedi ; the
paper being then meafured, fuppofe an inch fquare :
Now, as the objedi under the magnifier, which appear¬
ed to be one inch fquare, was in reality only ten hun¬
dredths, or the tenth of an inch, the experiment proves
that it is magnified ten times in length, one hundred
times in fuperfices, and one thoufand times in cube,
which is the magnifying power of the glafs ; and, in
the fame manner, a taole may be made oi the power of
all the other glafl’es.
In ufing the compound microfcope, the real fize of
the objedi is found by the fame method as in the fingle :
but to demonflrate the magnifying power of each glafs
to greater certainty, adopt the following method.—
Lay a two-feet rule on the ilage, and a micrometer
level with its furface (an inch fuppofe, divided into
IOO parts) i with one eye fee how many of thofe parts
are contained in the field of the microfcope, (fuppofe
50) ; and with the other, at the fame time, look 'for
the circle of light in the field of the microfcope, which
with a little pradlice will foon appear dillindl ; mark
how much of the rule is interfedfed by the circle of
light, which will be half the diameter of the field*
Suppofe eight inches ; confequently the whole diame¬
ter will be fixteen. Now, as the real fize of the field,
by the micrometers, appeared to be only 50 hundredths,
or half an inch, and as half an inch is only one 3 2d
part of 16 inches, it fhows the magnifying perver of the
glafs to be 32 times in length, 1024 fuperfices, and
32,768 cube (e).
63. Another way of finding the magnifying power
of compound microfcopes, is by ufing two micrometers
of the fame divifions ) one adjuffed under the magni¬
fier, the other fixed in the body of the microfcope in
the focus of the eye-glafs. Notice how many divi¬
fions of the micrometer in the body are feen in one
divifion of the micrometer under the magnifier, w hich
again muff be multiplied by the power of the eye-glafs.
Example : Ten divifions of the micrometer in the bo¬
dy are contained in one divifion under the magnifier ;
fo far the power is inertafed ten times : now, if the
eye-glafs be one. inch focus, fuel) glafs will of it-
felf magnify about feven times in length, which, with
the ten times magnified before, will be feven times ten,
or 70 times in length, 49CO fuperfices, and 343,000
cube.
“ If (fays Mr Coventry) thefe micrometers are em¬
ployed in the folar microfcope, they divide the objedi
into fquares on the fereen in fuch a manner as to render
it extremely eafy to make a draw ing of it. And (fays
he) I apprehend they may be employed to great advan¬
tage w ith fuch a microfcope as Mr A.dams’s lueernal ;
becaufe this inftrument mgy be ufed either by day or
night, or in any place, and gives the adlual magnifying
power without calculation.”
The
(e) It will be neceffary, for great accuracy, as well as for comparative obfervations, that the two-fect rule
fhould always be placed at a certain diftance from the eye : eight inches would, in general, be a proper d iff a nee.