MICROSCOPE. 
they are seen has any degree of convexity, 
the rays of light are directed from the cir- 
cumference towards tlie centre, in an angle 
proportional to the convexity of the glass, 
and meet in a point, at a greater distance 
from the glass, as it is less or more convex. 
This point, where the rays meet, is called 
the focus, which is nearer or further off, ac- 
coriling to the convexity of the glass ; for a 
small desree of convexity throws it to a 
considerable distance ; when the convexity 
is much, the focus is near. The magnifying 
power is in proportion to its convexity ; for, 
as a flat glass magnifies scarcely at all, the 
less a glass departs from flatness, the less it 
magnifies ; and the more it approaches to- 
wards a globular figure, the nearer its focus 
is, and the more its magnifying power. We 
shall refer to Optics for a more particular 
account of the principles, and proceed to 
describe some instruments of which we have 
made drawings. Plate Microscope, &c. 
The body of the microscope, A A, being 
a large tube, is supported by brass pillars, 
bb, rising from a wooden pedestal, c, in 
Which pedestal is a drawer to hold the ob- 
ject-glasses, and other parts of the appara- 
tus ; a lesser tube, e e, slides into the greater, 
and is moved up and down therein by a 
rack and pinion turned by a milled headed 
nut, t ; into this tube is pushed the body of 
the microscope, as shewn in the section 
fig. 1, and moving up and down with it, it 
has at its bottom another tube, much 
smaller than itself,/, with a male screw, g, 
at the end thereof, whereon to screw the ob- 
ject-glasses, or magnifiers : there are five of 
tliese magnifiers, numbered 1, 2, 3, 4, 5, 
which numbers are also marked on the inner 
tube, e e, to direct whereabout to place it, 
according to the magnifier made use of; 
but if it does not fit the eye exactly, move 
the inner tube gently higher or lower, by 
the nut t, till the object appears distinct. 
The greatest magnifiers have the smallest 
apertures, and the lower numbers. L, is a 
circular plate of brass, fixed horizontally 
between the three brass pillars, 6 b, and in 
the centre thereof a round hole. M, is 
adapted to receive a proper contrivance, 
fig. 4, for holding ivory sliders, wherein ob- 
jects are placed, which contrivance consists 
of a spiral steel wire, confined between 
three brass circles, one whereof is movea- 
ble for the admission of a slider. Q, is a 
concave mirror set in a box of brass, and 
turning in an arch, R, upon two small 
screws. From the bottom of the arch 
comes a pin, which being let down into a 
hole in the centre of the pedestal, enables it 
to turn vertically or horizontally, and reflect 
the light, either of a candle or tire sky, 
directly upwards on the object to be view- 
ed. V, is a plano-convex lens, which, by 
turning on two screws when the pin at the 
bottom of it is placed in the hole made for 
its reception in the circular plate, L, will 
transmit the light of a candle to illuminate 
any opaque object that is put on the round 
piece of ivory, or on ebony, for examina- 
tion, and it may be moved higher or lower 
as the light requires : this glass is useful to 
point the sunshine, or the light of a Candle, 
upon any opaque object; but in plain day- 
light is of no great use. The brass fish 
pan, fig. 3, is to fasten a smelt, gudgeon, or 
any such small fish upon, to see the blood 
circulate in its tail ; for which purpose, the 
tail of a fish must be spread across the ob- 
long hole, or the end of the pan, by slip- 
ping the button, on the backside of the 
pan, into a sUt, through the circular plate, 
L; a spring that is beneath the plate presses 
the button, and will make it steady, and 
present it well ; but if it be a frog, a newt, 
or eel, in which the circulation is desired to 
be shewn, a glass tube, fig. 6, is fittest for 
the purpose. The tail of a newt, or eel, 
or, in a frog, the web between the toe of 
the hind-feet, are the parts where it may be 
seen best. When the object is well ex- 
panded on the inside of the tube, slide the 
tube along under the circular brass plate, 
L, w'hcre there are two springs and a cavity 
made in the shank to hold it, and bring the 
object directly under the magnifier. 
There are three of these glass tubes, 
smaller one than another, and the size of 
the object must direct which to use ; but, 
in general, the less room the creature has 
to move about in, the easier it may be mag- 
nified, and the quieter it will lie to be exa- 
mined. Three loose glasses, viz. one plain, 
and two concave, belong also to the micro- 
scope, and are designed to confine objects, 
or place them upon occasionally. 
The long steel wire, fig. 7, (with a pair of 
plyers at one end, and a nob of ivory at the 
other, to hold fast, or lay objects on), slips 
backwards or forwards in a short brass 
tube, whereto a button is fastened, which fits 
into a little hole near the edge of the brass 
plate L, and then the object may be readily 
brought to a right position, and a light be 
cast upon it, either by the looking-glass un- 
derneath, or, if it be opaque, by the plano- 
convex lens, N. 
L, fig. 4, is a flat piece of ivdry, called a 
