OPT 
the focus of the lens in the Lieberkhun. They have a 
motion all ways by means of the fpring-focket a, the joint 
b, and the fhank c: they are placed with the pin c in the 
focket* of thefixedftage'R, fig. io. and d is afmall piece of 
ivory, to be placed upon the pointed end of the pliers ; 
it is black upon one fide, and white upon the other, to re¬ 
ceive opaque objects. 
R, fig. 4, is a Lieberkuhn of a larger fize than that firft 
mentioned, with a hole in its centre : this is fcrewed into 
the hole Q of a brafs ring, faftened to a long wire d, 
which moves up and down in the fpring-focket x of the 
ftage R, fig. io. in which it alfo moves fine ways; and thus, 
with the body AE above, forms an aquatic compound mi- 
crofcope forlhowing all forts of objeds in water and other 
fluids 'placed under it in a watch-glafs on the ftage. 
Sometimes a cone is ufed, with a proper aperture to ex¬ 
clude fuperfluous light, that would difturb a critical ob- 
fervation of a curious object; it is fcrewed to the under 
fide of the fixed ftage R. There is alfo whatis ufually 
called a bug-box, confining of a concave glafs with a plane 
one fcrewed over it; by means of which a bug, loufe, flea, 
&c. may be fecured and viewed alive. It is to be placed 
on the ftage R. 
Fig. 8. is the fifh-pan. In the long concave body ah, a 
fifh may be fo confined by the ribband cc, that the tranf- 
parent tail may be in part over the flit or hole at a. In 
this ftate, it is placed on the ftage R, with the pin d in the 
hole t of the ftage, and moves freely and horizontally for 
viewing the circulation of the blood, See. 
X, at the bottom of the Plate, is a brafs cell or but¬ 
ton, containing a very fmall lens, properly fet between 
two fmall plates of brafs, that it may be brought very 
near to the objed, when viewed with it as a fingle micro- 
fcope. This magnifier is fcrewed into the fame hole as • 
the wheel of fix magnifiers, Q, are in fig. 9. There is like- 
wife a lens, adapted to view and examine objects, by mag¬ 
nifying them fufficiently fo as to be able to apply them to 
the microfcope for infpedion : on this account it is called 
the cxplorutor. 
In ufing the microfcope, and while viewing objeds by 
either the fingle or compound inftrument, the focal dif- 
tances of the magnifiers are madeperfedly exad by turn¬ 
ing the pinion at the nut iv, in one way or the other, 
very gently in the teeth of the rack-work at the front of 
the bar FI. It is neceffary that the centres of the objed- 
lenfes or magnifiers, the ftage, and the mirrors at bottom, 
fhouhl all be in a right line in the axis of the microfcope, 
when opaque objeds are to be viewed, that are placed 
upon the ivory plate d, -fig. 5. or the forceps, and all other 
fuch forts of objedts which are placed in the centre of 
the ftage R, or the Aider-holder: but, when aquatic or 
living objeds, which require a great fpace to move in, 
are to be viewed, then the horizontal motion at ef, fig 10, 
is made ufe of, and the view may be extended laterally 
over the whole of the diameter of the objed or field of 
view, and, by putting the arm ahe forward or backward 
in its focket ef, the view is extended in the contrary di- 
redion equally well, and in this manner the whole of the 
objeds may be viewed without the leaft difturbance-. As 
the brafs arm abc may be brought to the height of three 
or four inches above the ftage R, fo by means of the 
rack-work motion of the ftage, a lens of a greater focal 
diftance than the greateft in the wheel Q may be occa- 
fionaliy applied in place of the wheel, and thereby the 
larger kind of objeds be viewed ; the inftrument be¬ 
coming, in this cafe, what is called a megulofcope. Two 
fizes of thefe lenfes, furnilhed with Lieberkuhns, are 
lliown at L, M. 
In viewing moving living objeds, or even fixed ones 
when nice motions are requilite, a rack-work and pinion 
is often applied to the arm abc: the arm is cut out with 
teeth ; and the pinion, as fhown at Y, is applied to work 
it. This ads but in one diredion 5 and, in order to pro¬ 
duce an equally-neceflary motion perpendicular to this, 
I C S. 619 
rack-work and pinion is applied tangent-wife to the ftage, 
which is then jointed. 
III. Reflecting - Microfcopcs. —The refleding microfcope 
is properly that which magnifies by refledion, as the 
above-mentioned ones do by refradion. 
The ftrudure of fuch a microfcope may be conceived 
thus: Near the focus of a concave fpeculum AB, Plate 
XII. fig. 1. place a minute objed C, that its image may 
be formed larger than itfelf in D j to the fpeculum join 
a lens, convex on both (ides, EF, fo as the image D may 
be in its focus. The eye will here fee the image inverted, 
but diftind, and enlarged ; confequently, the objed will 
be larger than if viewed through the lens alone. The 
inventor of this microfcope is the great fir I. Newton ; but 
the objeds appear dim in it. 
The conftrudion of this microfcope is more particularly 
explained in fig. 2. in which, inftead of the lens def, there 
is placed a fmall fpeculum def; fo that the objed acb 
being placed above it, at a little greater diftance from the 
focus g, has its image ACB formed by refledion, as in 
the other cafe it was by refradion, through the lens df. 
Now, if we fuppofethe focal diftance of the objed-fpecu- 
lum def and lens def the fame, the effed of the micro¬ 
fcope in other refpeds will be the fame alfo. For the dif¬ 
tance of the objed ab above the fpeculum, will be equal 
to the diftance of the objed ab below the lens, in order 
that the image may be formed at the fame diftance Ce. 
The pofition of the objed will be inverted; for all the 
rays flowing from the point a will be refleded by the fpe¬ 
culum to the point A, in the fame manner as if they came 
by refradion through the lens from the point a. Thus 
the part b in the objed will be refleded to the focus B in 
the image, which, therefore, is inverted. The power of 
magnifying will alfo be the fame in this and in the refled¬ 
ing telefcope of a fimilar conftrudion. For, fince the 
image AB and the objed ab are feen under equal angles 
from the vertex e of the fpeculum, the triangles aeb and 
AeB will be fimilar, and therefore AB : ab :: Ce : ce ; 
but, in the other, it is AB : ab :: Ce : ce. But the lat¬ 
ter ratio of thefe analogies is the fame in both; and, confe¬ 
quently, the firft is fo too. 
This microfcope is not fo eafy to manage as the com¬ 
mon fort. For vifion by refledion, as it is much more per- 
fed, fo it is far more difficult than that by refradion. 
Nor is this microfcope fo ufeful for any but very fmall or 
tranfparent objeds. For the objed, being between the 
fpeculum and image, would, if it were large and opaque, 
prevent a due refledion. In Dr. Smith’s refleding mi¬ 
crofcope there are two refleding mirrors, one concave and 
the other convex, and the image is viewed by a lens. To 
explain it: Let AD, fig. 3. be a large concave fpeculum, 
and ad a fmall convex one, each perforated in the middle 
with the holes BC, be. Both thefe are fegments of the 
fame fphere, or ground on tools of an equal radius, viz. 
of two inches, that fo the focrvl diftance of each fpeculum 
maybe juft one inch. Thefe two fpeculums are placed 
at the diftance of about if inch from each other, that fo 
an objed OPQ, being placed a little below the fmaller 
fpeculum, might be between the focus F and centre E of 
the larger fpeculum. Things thus circumftanced, the 
rays PA, PD, which flow from the point P in the objed, 
on the fpeculum AD, will be refleded towards a focus p, 
where an image opq would be formed, if the rays were 
not intercepted by the convex fpeculum 'ad; and the 
point p being nearer than its focus f, the rays A a, 
D d, which tend or converge towards it, will be refleded 
to a focus R, where the laft image, QRS, will be formed, 
to be viewed through the eye-glafs G, by the eye at I. 
This microfcope, though far from being executed in the 
beft manner, performed, Dr. Smith fays, nearly as well as 
the very beft refrading microfcopes; fo that he did not 
doubt but that it would have excelled them, if it had 
been executed properly. Dr. Smith’s own account of 
this inftrument may be feen in his Optics ; Remarks p. 94. 
1 IV. Solar 
