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14 MODERN MAGIC LANTERNS.
In the ordinary form of tray the jet is held by being slid
up or down on the pin of the tray and clamped by means of
the screw, s, Fig. 8. This arrangement, while it allows the
jet to be moved very readily and is simple and reliable, has
the disadvantages that the jet is supported by a point very
much behind its centre of gravity, the pin consequently
tending to lean forward, and that the operator who
manipulates the jet from behind has hold, as it were, of the
short end of a lever, the slightest movement of which displaces
the front of the jet very considerably. The arrangement
also is very easily disturbed by an accidental touch
at the back of the jet ; but, in spite of these drawbacks, this
form of tray is most often met with, and if substantially
made and used with proper care will be found to answer
every purpose. Mr. Andrew Pringle, a recognised authority
on lantern matters, uses in addition an arrangement of a
couple of jaws, which, when the jet has been adjusted in
position, grip it in front, and so help to keep it firm.
All jets are provided with a means of holding the lime for
the flame to play upon, and as the particular part of the
surface soon deteriorates and requires renewal, must have
also an arrangement for bringing a fresh portion of the lime
under the influence of the flame. This in most cases is
effected as shown in Fig. 8, where a vertical spindle, D,
carries a little circular table, E, through the base of which
the spindle projects, the lime being generally a perforated
cylinder in shape, which is slipped over the pin or spindle
and rested on the table. This pin is sometimes made long
enough to carry two limes, one on the top of the other ; this
may be useful for long displays, but we ourselves cannot
recall a time when it would have been a convenience. The
lime pin is connected at its base by means of bevel wheels,
F F, with another spindle, which ends in a milled head, G,
near the taps at the back of the jet ; the pin, however,
while turning with the wheels, is free to move up or down ;
and is indeed compelled to do so when the wheels revolve,
on account of a spiral thread which is cut upon it,
which engages in pins attached to the bracket carrying
the spindle. By this simple contrivance, which is not easily
indicated in the diagram, but which can be comprehended
THE LIMELIGHT. 15
in a moment on looking at a jet, by turning the milled head
G, Fig. 8, the lime pin, and with it the lime, not only revolves,
but at the same time moves up or down, so that a pencil
held upon the side of the lime would describe a spiral line
around it. The handiest form of screw for doing this is one
which raises the lime at least a quarter of an inch for each
complete revolution. A defect some forms of jet possess is
to be found in the fact that the screw is such a fine one that
when the lime has been turned completely round, the jet is
playing upon practically the same place on the lime as
that at which it started.
In some forms of jet, which now have almost gone out of
use, a flat disc of lime was employed instead of a cylinder,
as shown in Fig. 11. In this case the lime simply revolved
Fig. 11. BLOW-THROUGH JET WITH LIME DISC.
upon a central axis, each portion of a revolution sufficing to
bring a fresh part of it before the flame.
The limes for limelight should be turned true, not only as
regards their outer surface, but the hole through the centre
should be central also, since if it is not, the distance between
the jet and the lime will vary as it is turned. As this
distance is an important factor in getting the most light
which a jet is capable of giving, it follows that if one position
of such a lime is correct, on turning it, the distance is altered
and the light diminished In this connection, we may observe
that what are known as "Excelsior "limes are as a rule better
shaped than their rivals, the " Nottingham " limes, although
for the mixed jet the latter are preferable when well formed.
Limes are often spoken of as hard or soft, and these two