Untitled

42 MODERN MAGIC LANTERNS. JETS. 43
anything go wrong, the oxygen should at once be turned off.
The roaring may also be caused by the lime being at an
unsuitable distance from the nozzle, or from it being pitted,
in which cases the remedies are obvious.
It will be found that with this form of jet the lime will
require turning nearly every minute ; with a blow-through
this is not so frequently necessary. The correct distance
between the lime and the nozzle of a mixed jet is about oneeighth
of an inch. It is possible to use a mixed jet with the
coal gas supply drawn direct from the main, but it is not at
all advisable to attempt this, two cylinders should invariably
be used.
Mr. Pringle is the inventor of an attachment to the jet,
which is shown in Fig. 10, in which two taps are seen in the
middle of the gas tubes, actuated from the back of the
lantern by means of a rod and bevel wheels. These taps
being left open, and the jet adjusted in the usual manner, if
it is required to turn it out for a little while, it can be done
with the knob, which turns the oxygen completely off and
leaves a little coal gas passing to keep the jet alight. When
the light is wanted again the knob is turned back and the
jet is once more burning properly without having to be
readjusted, as would be the case had it been extinguished in
the usual way by means of the taps used for adjustment.
This "cut off," as it is called, is also handy for lowering
the light a little when necessary, since both gases are cut
off in proportion.
The form of mixing chamber shown in Fig. 32 does not
answer when an ether saturator is used in place of coal gas,
on account of the danger of a light passing back into the
saturator and causing an explosion. For this purpose, therefore,
a jet is employed in which the mixing chamber is
packed with pumice or other material designed to act on the
same principle as the gauze in the miner's Davy lamp, and
to prevent any flame passing through it.
The mixed jet yields a much more concentrated spot of
light than the blow-through--an advantage optically. This is
due to the smallness of the flame, which cannot be increased
with advantage. A feature of all jets is that by increasing
the bore of the nipple the amoun' of light is not increased
unless the pressure is greater also, and even then only
within certain limits, a point being soon reached beyond
which such increase becomes both wasteful and noisy.
It will be noticed that the jet plays upon the lime at an
'angle, and the amount of this angle affects to a great extent
the amount of illumination. If the nozzle is at a very slight
angle to the lime, that is, if it is nearly vertical, much of its
heating power will be lost, if the angle is too great the
shadow of the nozzle will be thrown upon the condenser.