OPTICS. 
(■■ G 
jefts may be fixed in the nippers M, or (luck on the point 
N, and then brought under the eye-glaffes ; or they may 
be laid on one oh the glaffes which.fit the ftage. The ap¬ 
paratus to this inftrument confifts of three ivory lliders ; 
a pair of nippers; a pair of forceps; a flat glafs and a con¬ 
cave ditto, both fitted to the ffage. 
The two lad microfcopes are frequently fitted-up with 
a toothed rack and pinion, for the more ready adjuffment 
of the glaffes to their proper focus. 
Dr. Withering, in his “ Botanical Arrangements,” de- 
fcribes a portable botanic mievpfcope. It confilis of three 
brafs plates, A, B, C, fig. 12, which are parallel to* each 
other ; the wires D and E are rivetted into the upper and 
lower plates, which are by this means united to each 
other; the middle plate or ftage is movable on the afore- 
laid wires by two little fockets which are fixed to it. The 
two upper plates each contain a magnifying lens, but of 
different powers; one of thefe confines and keeps in their 
places the fine point F, the forceps G, and the fmall 
knife H.—To ufe this i n It rumen t, unfc re w the upper lens, 
as it appears in the figure, and take out the point, the 
knife, and the forceps then fcrew the lens on again, 
place the objeft on the ftage, and then move it up or 
down till you have gained a diftinft view of the objeft, as 
. one lens is made of a Ihorter focus than the other ; and 
iparedenfes of a fcill deeper focus may be had if required. 
This little microfcope is the moll: portable of any. Its 
principal merit is its iimplicity. 
Mr. B. Martin has contrived to mount feveral lenfes in 
one frame, which are convenient for various purpofes, 
and are carried in the pocket. He calls this apparatus a 
hand meghlafcope, from its ufe in viewing the larger fort of 
fmall olijeft's expeditioufly. The cafe, with its three frames 
and lenfes, is repreferited in fig. 13. The lenfes are com¬ 
monly of 1, ig, and 2, inches focus ; they are contrived io 
as to turn over each other, and Ihut into the cafe. The 
three lenfes fingly afford three magnifying powers ; and, 
by combining two and two, we obtain three more; D with 
C making a fourth, D with F a fifth, and C with F a iixth ; 
and all three combined together make another; fo that 
by this Ample apparatus we have (even different piagni- 
fying powers. When the three lenfes are combined, it is 
better to turn them in, and look through them by the 
fmall apertures in the hides of the cafe. The eye in this 
cafe is not incommoded by external light; the aberration 
of the fuperfluous rays through the glaffes is precluded ; 
and tile eye coincides more exactly with the common axis 
of the lenfes. 
A very ufeful and eafy kind of microfcope (defcribed 
by Joblot,and which has been long in ufe), adapted chiefly 
for viewing, and confining at the fame time, any living 
infefts, fmall animals, See. is (flown at fig. 14.. where A 
reprefents a glafs tube, about if inches diameter, and 2 
inches high. B, a cafe of brafs or wood, containing a 
Hiding tube, with two or three magnifying glaffes that 
may be ufed either feparately or combined. In the in- 
fide, at the bottom, is a piece of ivory, black and white 
on oppofite Tides, that is occafionally removed, and ad¬ 
mits a point to be ferewed into the centre. The cap un- 
ferews at D, to admit the placing of the object; the pro¬ 
per diftance of the glaffes from the objeft is regulated by 
pulling up or down the brafs tube E at top containing 
the eye-glafl'es. This microfcope is particularly ufeful 
for exhibiting the well-known curious Curculio imperia- 
lis, vulgarly called the diamond beetle, to the gfeateft ad¬ 
vantage; for which, as well as for other objefts, a glafs 
bottom, and a polilhed refleftor at the top, are often ap¬ 
plied, to condenfe the light upon the object. In this cafe, 
the Hand and brafs bottom F, as (hown in the figure, are 
taken away by unferewing. 
Lyonet's anatomical difj'eSling microfcope, invented by 
that gentleman for the purpofe of minute diffeftions and 
microlcopic preparations. This inftrument muff be truly 
ufeful to amateurs of the minutite of infefts, See. being 
the belt adapted of any for the purpofes of difleftion. 
With this inftrument Mr. Lyonet made his very curious 
microfcopical diffeftion of the chenille de .faille, as related 
in his Traite Anatomique de la Chenille qui ronge le 
bois de faule. AB, fig. 15, is the anatomical table, which 
is fupported by a pillar NO ; this is ferewed on the foot 
CD. The table AB is prevented from turning' round by 
means of two fteady pins. In this table or board there 
is a hole G, which is exactly over the centre of the mir¬ 
ror EF, that is to reflect the light on the objeft:; the 
hole G is defigned to receive a flat or concave glafs,, on 
which the objefts for examination are to be placed. 
RXIZ is an arm formed of feveral balls and fockets, by 
which means it may be moved in every poflible fituation ; 
it is fixed to the board by means of the (crew H. The 
laft joint IZ has a female fcrew, into which a magnifier 
may be ferewed as at Z. By means of the fcrew H, a 
fmall motion may be. occafionally given to the joint IZ, 
for adjlifting the lens with accuracy to its focal diftance 
from the objeft. Another ch'ain of balls is fometimes 
ufed, carrying a lens to throw light upon the object; the 
mirror is likewife fo mounted, as to be taken from its 
place at K, and fitted on a clamp, by which it may be fixed 
to any part of the table AB. 
To ufe this inftrument: Let the operator fit with his 
left fide near a light window ; the inftrument being placed 
on a firm table, the'fide DH towards the ftomach, the 
obfervations fnould be made with-the left eye. Jn diffeft- 
ing, the two elbows are to be fupported by the table on 
which the inftrument reft's, the hands refting againft the 
board AB ; and, in order to give it greater liability (as a 
fmall (hake, though imperceptible to the naked eye, is 
very vilible in the microfcope), the diffefting-inftruments 
are to be held one in each hand, between the thumb and 
two fore-fingers. 
II. Double or Compound Microfcopes. —Doublemicrofcope 
are fo called, from being a combination of two or raor 
lenfes. The particular and chief advantages which th 
compound microfcopes have over the Angle, are, that th 
objefts are reprefented under a larger field of view, an 
with a greater amplification of reftefted light. But it i. 
to be obferved, that compound microfcopes fometimes 
exhibit a fallacious appearance, by reprefenting convey 
objefts concave, and vice verja. See Phil. Tranf. N° 476. 
To the induttry and addrefs of our countryman Mr. 
Marjhal we are indebted for the original double micro¬ 
fcope. It is reprefented on Plate XI. at fig. 1. The eye- 
glafs isferewed in at W, the objecl-glafs at C, the middle 
glafs at A. B is the cover or lid, to keep out the duff 
from the eye-glafs W ; X is the place of the eye, W a> 
fcrew where the eye-glafs lies; U, the drawer, where the 
outermoft tube AU is disjoined from the inner one, of 
the fame length ; Z, the frame or bafis on which the 
microfcope (lands firm ; T, a fmall drawer in the frame 
or bafis, with a ledge or till in it, having fix partitions 
to hold fo many feveral objeft-glaffes, one magnifying 
more than another, and fixed in brafs cells ready to fcrew 
on at C ; the other part of the drawer ferves to hold the 
objeft-plate ; a pair of fmall nippers, to take up or handle 
any objeft conveniently; another objeft-plate, having 
one fide white and the other black, to fix your objefts 
upon, as black objefts upon white, and white objefts on 
black. LM is a brafs ball and locket, on which the 
whole body of the microfcope is movable, fo as to lie in 
anypofition for the light. LK, a fquare brafs pillar, on 
which the microfcope is movable up and down, by means 
of the collar E, into which the arm D (holding the mi¬ 
crofcope) is continued. G, another brafs collar- Hiding 
up and down on the pillar LK, having a fmall fcrew H, 
by which it is, as occasion ferves, fixed fall to the faid 
pillar at any height. I, a large brafs nut, in whofe cen¬ 
tre is a female fcrew fitted to the male fcrew F, which is 
fixed in the collar E ; by the turning of which nut I (the 
collar G being firff fixed to the pillar by the fcrew H) the 
microfcope is raifed up or down on the pillar, and made 
to come nearer or go farther from the objeft Pc; and, 
4 which 
