Untitled

72 MODERN MAGIC LANTERNS.
A detail which may be mentioned here is that of the
means of communication between the lanternist and lecturer
when separated in a large hall. Some people use an electric
bell, some a reading lamp with a little red glass window
which can be uncovered, others an ordinary bell, or they
give a slight tap to indicate when another slide is wanted. Any
of these methods can be employed, but it is advisable, when
possible, to use one which shall not attract the attention of
the audience ; for this reason an electric bell, without the
bell itself, giving, therefore, a slight buzzing sound instead
of a ring, will be found best.
Owing to a recent accident, the London County Council
have turned their attention to the use of the limelight in
places of public entertainment, and have laid down the
following regulation, which is usually insisted upon by them:
" That proper tanks be provided, placed in ventilated,
"brick-built chambers, fitted with iron doors and frames ;
"that the hydrogen and oxygen gases be placed in separate
" chambers ; that the screws to the holders of each gas be of
"different diameters ; that *stopcocks be fixed at the lenses
"and to the supply pipes on the platform level ; and that
"flexible iron tubing with screw connections be used."
The regulations themselves, in the opinion of most experts,
are anything but satisfactory, that which insists upon the
use of flexible iron tubing in place of india-rubber being
positively dangerous if fully acted upon. That clause
dealing with the screws of the gas holders is a wise precaution,
and is now universally adopted ; and it is no doubt
an excellent thing to have the gas cylinders outside the
place of meeting. On the other hand, insistence upon the
storage of oxygen, even when in a pneumatic holder, in
"ventilated, brick-built," etc., is vexatious, when it is
remembered that the gas in such a form is perfectly
harmless. Still, those who use the limelight in public,
within the jurisdiction of the London County Council, have
got to submit to the regulations until they are amended, and
had better, therefore, know exactly what it is they require.
"Much amusement has been got out of the rule which commands stopcocks
to be fitted at the lenses, the Council probably mean at the jet, judging
the context.
CHAPTER XII.
he Manipulation of the lantern.
WHEN the screen and lantern have been erected in their
proper positions, the adjustment of the one to the other has
to be taken in hand.
The first point to receive attention should be that the
position of the screen is such that it is exactly at right
A
Fig. 58. DIAGRAM SHOWING THE CORRECT POSITION OF THE SCREEN.
angles to the axis of the lantern. This is shown in Fig. 58, in
which A c is the axis of the lantern, that is, an imaginary line
drawn from the centre of the condenser, through the centre
of the objective, meeting the screen B B at the point c; the
angles B C A should all be right angles. If they are notthat
B