MICROSCOPE. 
slider, with four round holes through it, and 
objects placed in them, between Muscovy 
talcs, or isinglass, kept in by brass wires. 
It is proper to have a number of these 
sliders filled with curious objects, always 
ready, as well as some empty ones, for any 
new thing that offers. When made use of, 
thrust them between the brass rings of the 
contrivance on purpose for them, as shewn 
in fig. 4, which shoots into the round hole 
M, in the centre of the brass plate, L ; this 
keeps them steady, and, at the same time, 
permits them to be moved to and fro for a 
thorough examination. 
The upper part of Plate Microscope, &c. 
describes the construction of a microscope, 
used for observing the divisions on mathema- 
tical instruments, and subdividing them into 
smaller portions. The drawing was made 
from one of those used by Mr. Troughton in 
his instruments; and the position of foifl- of 
them are shewn in the drawing of one of his 
astronomical circles, see Circular Instru- 
ment, fig. 1, an elevation sideways of the 
microscope; fig. 2, a section in the same 
direction ; fig. 3, a section endways, and 
four and five parts shewn separately. 
The microscope is fixed to the instru- 
ment it is applied to, by two nuts, a a, figs. 
1 and 2, which work upon a male screw, cut 
on the external tube ; these nuts have a 
smaller part turned upon them, which ex- 
actly fit into a circular hole in a piece of 
brass fixed to the instrument, and by screw- 
ing the nuts tight, the microscope is fixed, 
as shewn in the drawing of the astrono- 
mical circle before referred to. B, fig. 
1 and 2, is the 'sliding tube containing two 
convex lense.s, 6 d, fig. 2, whose combined 
foci fall upon the wires to be hereafter de- 
scribed ; this tube slides in another, D, 
screwed into a thin, square bojp, E, which 
contains the wires and screw, and it is 
shewn opened in fig. 3; it has a square 
frame, fig. 6, sliding in it ; to this is affixed 
a very fine screw, which comes through the 
top of the box, and has a nut, F, screwed 
on it ; at e, within the box, is a spring, form- 
ed of steel wire, acting upon the frame, fig. 
6, so as to draw it into the box*, by this the 
shoulder of the nut, F, is forced down upon 
the top of the box, and all shake or loose- 
ness in the motion prevented. 
The frame has two exceedingly fine wires 
stretched across it, as in the figure, and it 
is by these the divisions on the instrument 
are observed : G is a conical tube, screwed 
into the principal tube of the instrument, 
with the object-glass at- its end : the box, 
VOL. IV. 
E, also contains a thin brass plate, fig. 4, 
wdiich slides in it benfeath the frame, fig. 5, 
and is moved when necessary by a small 
screw, a, going through the bottom of the 
box, whose action is counteracted by two 
thin slips of watch-spring, (seen in fig. 3) ; 
this plate has an oblong hole through it to 
see through, and on one side it is cut into 
fine notches, at such a distance apart that 
one turn of the nut, F, when viewed through 
the eye-tube, moves the cross wires in the 
frame, fig. 5, exactly one of these notches, 
and by that means the notches register the 
number of revolutions the nut has made. 
In adjusting and adapting this microscope to 
any instrument, the sliding eye-tube, B, is 
to be slid in or out of the tube, D, till the 
cross wire in the frame, fig. 5, are seen per- 
fectly distinct. The microscope is then to 
be placed in such a position on the iustrii- 
raent it is applied to, that the line of divi- 
sions on the arc shall be parallel to the mo- 
tion of the cross wires and frame, fig. 5, and 
screwed into its holder by the nuts, a a, (as 
shewn in the drawing of the circle) and the 
focus is adjusted so as to see the divisions 
on the arc distinctly, by turning these 
screws backwards or forwards, and moving 
the whole microscope nearer to, or further 
from the arc, until it is adjusted to distinct 
vision. The operator then looks through 
the microscope, and observes whether one 
division on the divided arc of the instrument 
answers to twenty of the notches of the 
scale, fig. 4, (which will each be then equal 
to one minute) if not the conical tube, G, 
containing the object-glass at its end, must 
be screwed in or out of the body of tire mi- 
croscope, until the image of the division or 
degree is lengthened or shortened, till it 
does, and a loose nut on the tube holds it 
at any place required ; if this adjustment 
deranges the other before made for distinct 
vision, it must be rectified by the nuts, a a, 
and if this alters the space measured by the 
scale, fig. 4, the object-glass must be alter- 
ed, and then again the nuts, a a, first one 
and then the other, until botli adjustments 
of measure and distinct visiou are perfect. 
The small screw which gives motion to the 
scale, fig. 4, is used to adjust it to the point 
of commencement on the circle or divided 
arc ; or when two opposite microscopes are. 
applied to the same circle,' to adjust them 
so that they shall both read alike, that is, 
so that a line between them shall pass 
through the centre of the circle, and for the 
same purpose the small divided circle, x, of 
the nut, F, will slip round upon the nut 
Ff 
