Place the focket, which is at the bottom o f the opake ftage,
on the bar O X T , fo that the concave mirror o may be next the
end D E o f the wooden body.
Screw the tubes P O into the end D E. The magnifier you intend
to ufe is to be fcrewed on the end o of thefe tubes.
The handle G b, or milled nut, Fig. 2, mull be placed on the
fquare end o f the. pinion a.
Place the lamp lighted before the glals lump n, and the objeft
you intend to examine between the fpring plates, o f the ftage,
and the inftrument is ready for ufe.
In all microfcopes, there are two circumftances which mull be
particularly attended to ; the modification of the light, or the
proper [quantity to illuminate the object; fecondly, the adjuft-
ment o f the inftrument to the focus o f the glaffes and the eye o f
the obferver. In the ufe of the lucemal microfcope there is a
third circumftance, which is, the regulation o f the guide for the
eye, each of which I {hall confider by itfelf. l.
l . T o throw the light upon the objeft. The flame of the lamp
is to be placed rather below the center o f the glafs lump n, and
as near it as poflible; the concave mirror o mull be fo inclined and
turned, as to receive the light from the glafs lump, and reflect it
thence upon the object; the belt fituation o f the concave mirror,
and the flame o f the lamp, depends on a combination of circumftances,
which a little practice will difcover.
2. T o regulate the guide for the eye, or to place the center of
the eye-piece L , fo that it may coincide with the focal point of
the lenfes, and the axis of vifion. Lengthen and Ihorten the
tubes M N by drawing out or pufhing in the inner tube, and
raifing or depreffmg the eye-piece M L, till you find the large
lens (which is placed at the end A B o f the wooden body) filled by
an uniform field o f light, without any prifmatic colours round the
edge; for till this piece is properly fixed, the circle o f light will
be very fmall, and only occupy a part o f the lens: the eye mull
be kept at the center of the eye-piece L, during the whole o f the
operation; which may be rendered fomewhat eafier to the obferver,
on the firftufe-of the inftrument, if he holds a piece o f white
paper parallel to the large lens, removing it from, or bringing it
nearer to them, till he finds the place, where a lucid circle, which
he will perceive on the paper, is brighteft and moft did inti, then
to fix the center of the eye-piece to coincide with that fpot; after
which a very fmall adjuftment will let it perfectly right.
3. T o adjuft the lenfes to their focal diftance. This is effected
by turning the pinion a, the eye being at the fame time at the eyepiece
L . I often place the grey glafs before the large lenfes,
while I am regulating the guide for the eye, and adjufting for
thé focal diftance.
If the obferver, in the procefs o f his examination of an objea,
advances rapidly from a {hallow to a deep magnifier, he will fave
himfelf fome labour by pulling out the internal tube at O.
K 2 The