20 
THE YOUNG SCIENTIST. 
is far preferable. This tool is represented 
half size in Fig. 1, and scarcely needs ex- 
planation ; it has a hollow screwed mandril 
and T-rest, and is held in the vice by the 
tongue at the bottom. The pulley has 
three speeds, the smallest of which is § 
inch in diameter: it should also have a 
is of a very simple construction ; there are 
two screws with milled-heads, marked A, 
(one of which is shown at A, Fig. 2, the 
other being on the opposite side, and seen 
in Fig. 3), for the purpose of adjusting the 
cutter to the diameter of a cell required to 
receive the lenses. These screws act in 
Fig. 2.— LATHE AND SLIDE BEST. 
socket for carrying a fixed magnifier, 
under which the minutest lenses are 
turned. The best bow is an old fencing 
foil ground down so as to be very thin and 
light. Catgut does not answer well for 
the string, as it soon gets frayed out over 
the small pulley. I have found the best 
packing twine preferable. During work 
this is kept slightly moist, and rubbed 
with a piece of soap ; in this way a length 
of it will outlast a day's work, especially 
if a little more twisted before it is attached 
to the wire hook at the top of the bow. 
A surplus stock of string may be wound 
about the guard, just above the handle, so 
that it can be drawn out as required." 
In regard to the best lathe for mounting 
lenses, Mr. Swift contributes the follow- 
ing note to Dr. Beale's, work on the mi- 
croscope : 
"The tool best suited for mounting the 
lenses in their cells is shown in the accom- 
panying engraving (Fig. 2) ; this can be 
worked with a drill bow, or can be made to 
act with the foot, in the way of an ordinary 
lathe, which is preferable, as it leaves both 
hands at liberty for manipulating with the 
slide-rest carrying the cutter. The slide- 
rest, as will be perceived by the engraving, 
opposite directions upon a dove-tail slide, 
moving at right angles to the mandril of 
the lathe. B is a bar of metal, about 6 
inches long, upon which the cutter is 
fixed. This bar is planed out A shape, 
and bears upon two supports of similar 
form. By this means a sliding motion is 
obtained, in a direct line with the axis of 
the mandril, upon which the object-cell is 
screwed. By moving the milled-head 
screw in Fig. 3, the point of which bears 
against a stop, any depth of cut required 
for the combinations can be obtained. 
The bar B, Fig. 2, when in use, must be 
held firmly on its fitting, and pushed 
along until the cut ceases by the point of 
the milled-head screw coming in contact 
with the stop before mentioned. Thus the 
required depth of the cell can easily be 
obtained. It is best, before proceeding to 
mount the combinations in their cells, to 
see that the mount takes a good bearing 
against the chuck in which it is screwed. 
If this precaution is not taken it often 
happens, after the objective is nearly fin- 
ished, that the lenses first mounted are 
found to run untrue, owing to the main 
screw of the mount taking a fresh bearing 
by the continued screwing and unscrew- 
