convex lens, that is mounted in a femicircle and fixed on a Hand,
fo that it’s pofition may be eafily varied; place this lens between
the objeft and the window, fo that it may collect a conliderable
number o f the folar rays, and refract them on the objeft, or the
mirror o f the microfcope. I f the light thus colleded from the
fun is too powerful, it may be tempered, by placing a piece o f
oiled paper, or a glafs lightly greyed, between the objeft and the
lens: by this means, a convenient degree o f light may be obtained,
and diffufed in an equal manner over the whole furface o f an.
objeft, a circumltance that {hould be particularly attended to ;
for if the light be thrown in an irregular manner, that is, larger
portions o f it on fome parts than on others, it will not be
diftinftly exhibited.
Where the folar light is preferred; it will be found very convenient
to darken the room, and to refleft the rays of the fun on
the above-mentioned lens,by means o f the mirror o f a folar microfcope
fitted to the window-Ihutter; for by this apparatus the-
obferver will be enabled to preferve the light on his objeft, nat-
withftanding the motion of the fun.
Cutting off the adventitious light- as much as poffible, by
darkening the room where you are ufing the microfcope, and
admitting the light only through a hole in the window-fhutter, on
at moft keeping one window only open, will alfo be found very
conducive towards producing a diftinft view of the objeft.
As the motion o f the fun, and the variable ftate o f our atmo—
fphere, renders folar obfervations both tedious and inconvenient,,
it will be proper for the obferver to be furnifhed with a large tin;
lhnthorn,
M i c r o s c o p i c a l E s s a y s . 133
lanthorn, made fomething like the common magic lanthorn, fit
to contain one of Argand’s lamps.* The lanthorn fhould have
an aperture in the front, that may be moved up and down, and
capable of holding a lens; by this a pleafing uniform denfe light
may be eafily procured. The lamp fhould move on a rod, that
it may be readily elevated or depreffed. The lanthorn may be
ufed for many other purpofes, as for viewing o f piftures,. exhibiting
microfcopic objefts on a fcreen, &c.
Many tranfparent objefts are feen beft in a weak light; among
thefe we may place the prepared eyes of flies and animalcula in-
fluids; the quantity o f light from a lamp or candle may be
Leflened by removing the microfcope to a greater diftance from
them, or it may he more effeftually Ieffened by cutting off a part
©f the cone o f rays that fall on the objeft, either by placing the
cone, Fig. E, Plate IX. under the ftage, or by forming circular
apertures o f black paper, o f different fizes, and placing either
a larger or fmalfer one on the refleftmg mirror, as occafion
may require.
There is an oblique fituation o f the mirrors, and confequently
o f the light, which is eafily difcoveredby praftice, but for which;
no general rule can be given, that will exhibit an objeft more
beautifully and more diftinftly than any other, pofition, fliewmg
rite furface, as well as, thofe parts through which the light »
tranfmitted..
Abetter
* The lamp (hould not Be of'the fountain kind, becaufe the rarefaffion of the
ait in,the lanthorn will often force the oil over.