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In this manner lie was able, by tlie 
help of a ruler, divided into inches and 
small parts, and laid on the pedestal of the 
microscope, as it were to cast the magnified 
appearance of the object upon the ruler, 
and thus exactly to measure the diameter 
which it appeared to have through the glass ; 
which being compared with the diameter as 
it appeared to the naked eye, easily shewed 
the degree in which it was magnified. A 
little practice, says Mr. Baker, will ren- 
der this method exceedingly easy and plea- 
sant. 
Mr. Martin, in his Optics, recommends 
such a micrometer for a microscope as had 
been applied to telescopes ; for he advises 
to draw a number of parallel lines on a 
piece of glass, with the fine point of a dia- 
mond, at the distance of one-fortieth of an 
inch from one another, and to place it in 
the focus of the eye-glass. By this method 
Dr. Smith contrived to take the exact 
draught of objects viewed by a double mi- 
croscope ; for he advises to get a lattice, 
made with small silver wires or squares, 
drawn upon a plain glass by the strokes of a 
diamond, and to put it into the place of the 
image formed by the object glass. Then, 
by transferring the parts of the object, seen 
in the squares of the glass or lattice, upon 
similar corresponding squares drawn on 
paper, the picture may be exactly taken. 
Mr. Martin also introduced into compound 
microscopes another micrometer, consisting 
of a screw. 
A very accurate division of a scale is per- 
formed by Mr. Coventry, of Southwark. 
The micrometers of his construction are 
parallel lines drawn on glass, ivory, or 
metal, from the 10th to the 10,000th part of 
an inch. These may be applied to micro- 
scopes for measuring the size of minute ob- 
jec's, and the magnifying power of the 
glasses ; and to telescopes for measuring 
the size and distance of objects, and the 
magnifying power of the instrument. To 
measure the size of an object in a single 
microscope, lay it on a micrometer whose 
lines are seen magnified in the same propor- 
tion with it, and they give, at one view, the 
real .size of the object. For measuring the 
magnifying power of the compound nrierd- 
jBcope, the best and readiest method is the 
following : On the stage, in the focus of the 
object-glass, lay a micrometer, consisting of 
an inch divided into 100 equal parts, count 
how many divisions of the micrometer are 
taken into the. field of view; then lay a two- 
foot rule parallel to the micrometer; fix 
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one eye on the edge of the field of light, and 
the other eye on the end of the rule, whieli 
move till the edge of the field of light and 
the end of the rule correspond ; then the dis- 
tance from the end of the rule to the middle 
of the stage will be half the diameter of the 
field. Ex. gr. If the distance be 10 inches 
the whole diameter will be 20, and the 
number of the divisions of the micrometer 
contained in tlie diameter of the field is the 
magnifying power of the microscope. 
Mr. Adams has applied a micrometer 
that instantly shews the magnifying power 
of any telescope. 
In the Philos. Trans, for 1791, a very 
simple scale micrometer, for measuring 
small angles with the telescope, is described 
by Mr. Cavallo. This micrometer consists 
of a thin and liarrow slip of mother of-pearl 
finely divided, and placed in the focus of 
the eye-glass of a telescope, just where the 
image of the object is formed; whether 
the telescope is a reflector or a refractor, 
provided the eye-glass be a convex lens. 
This substance, Mr. Cavallo, after many 
trials, found much more convenient than 
either glass, ivory, horn, or wood, as it is a 
very steady substance, the divisions very 
easy marked upon it, and when made as 
thin as common writing paper it has a very 
useful degree of transparency. 
MICROPUS, in botany, a genu,s of the 
Syngenesia Polygamia Necessaria class and 
order. Natural order of .Coinpositas Nu- 
camentace®. Corymbifera;, Jussieu. Essen- 
tial character : calyx caliclcd ; ray of the 
corolla none: female, florets wrapped up 
in the calycine scales; down none; recep- 
tacle chafiy. There are two species, viz. 
M. supinus, trailing micropus ; and M. 
erectus, natives of Spain and the Levant. 
MICROSCOPE, in optics. By micro- 
scopes are understood instruments of what- 
ever structure or contrivance, that can 
make small objects appear larger than they 
do by the naked eye. This is effected by 
means of convex glasses. When only one 
convex glass or lens is used, the instrument 
is called a single microscope ; but if two 
or more are employed conjointly to mag- 
nify objects, it is then called a double or 
compound microscope. When objects are 
seen through a perfectly flat glass, the rays 
of light pass through it from them to the 
eye, in a straight direction, and parallel to 
each other, and consequently the objects 
appear very little either diminished or en- 
larged, or nearer or further off than to the 
naked eye. But if the glas.s through which 
