IF you advance towards M, half the focal diftance, the apparent
diameter will be only increafed one-third. If, on the contrary,
the point o f fight is lengthened to double the diftance o f it’s
focus, then the magnified diameter will appear to be three times
that o f the real objeft. Mr. Magny concludes from hence, that
there is an impropriety in eftimating the magnifying power of the
eye glafs of compound microfcopes, by feeing how often it’s focus
is contained in eight or ten inches ; and to obviate thefe defeffs,
he recommends two methods to be ufed, which reciprocally confirm
each other.
The firft and moft fimple method to find how much any compound
microfcope magnifies an objedt, is the fame which is
defcnbed by Dr. Hooke in his. Micrographia, and is. as follows..
place an accurate fcale, which is divided into very minute parts,
©fan inch, on the ftage o f your microfcope-; adjuft the microfcope,
till thefe divifions appear diftinft ; then obferve with the
other eye how many divifions o f a rule, fimilarly divided and
held at the ftage, are included in one o f the magnified divifions :
for i f one divifion, as feen with one eye through the microfcope,
extends to thirty divifions on the rule, which is feen by the naked;
eye, it is evident, that the diameter o f the objeft is increafed oj-
magnified thirty times.
For this purpofe, we often ufe a fmall black ebony rule, (fée
Fig. 4, Plate II. A,) three or four tenths of an inch broad, and-
about {even inches long ; at each inch is fixed a piece of ivory,
the firft inch is entirely of ivftry, and fubdivided into ten equal
parts.
2. A
2. A piece o f glafs, Fig. 2, fixed in a brafs or ivory {lifer; on
the diameter o f this are drawn two parallel lines, about three-
tenths o f an inch long; each tenth being divided, one into three,
the fecond into four, the third into five parts. T o ufe this, place
the glafs, Fig. 2, on the middle o f theftage, and the rule, Fig. 4,
on one fide, but parallel to i t ; then look into the microfcope
with one eye, keeping the other open, and obferve how many
parts one-tenth of a line in the microfcope takes in upon the
parts o f the rule feen by the naked eye. For inftance, fuppofe
with a fourth magnifier, that one-tenth o f an inch magnified
anfwers in length to forty tenths or parts on the rule, when feen
by the naked eye, then this magnifier increafes the diameter of
the objeft forty times.
This mode o f aftual admeafurement is, without doubt, the
moft fimple that can be ufed; by it we comprehend, as it were,
at one glance, the different eftefls o f combined glafles-; it faves
the trouble, and avoids the obfcurity that attends the ufual modes
o f calculation; but many perfons find it exceedingly difficult to
adopt this method, becaufe they have not been accuftomed to
obferve with both eyes at once. We {hall therefore proceed to
defcribe another method, which has not this inconvenience.
O f the Needle M icrometer.
Fig. 8, Plate II. A , reprefents this micrometer. The firft of
this kind was made by my father, and was defcribed by him in
his Micrographia Illuftrata. It confifts o f a fcrew,. which has
fifty threads to an inch; this fcrew carries an index, which points
to the divifions on a circular plate, which is fixed at right angles
H2 to