THE ILLUMINANT
The first necessity for lantern projection is a strong light, and this can
be obtained from a variety of sources, the principal means in common use
being approximately in order of excellence as follows: paraffin-oil,
incandescent spirit, incandescent gas, acetylene, acetylene air blast,
oxyhydrogen (limelight), oxyether, and electric light in its various forms.
The ideal characteristics to be sought for are (1) great intrinsic
brilliancy; (2) minimum _size_ of luminous spot; (3) freedom from flicker;
(4) freedom from smell; (5) absence of any preponderating colour; (6)
cheapness; and (7) convenience. There is no question whatever as to which
of the available sources of light most perfectly combines all the above if
it is available, viz. the electric arc. If a current supply is in {4} the
building, this form of lighting easily excels all others, except possibly
in the matter of flicker, and even in this respect there is very little
fault to be found with it.
From all other points of view it is wellnigh perfect, inasmuch as it
provides an extremely concentrated and intensely luminous spot, of almost
perfect whiteness (if anything slightly bluish), no smell, comparatively
little heat, convenient and inexpensive. So great is the advantage of the
electric arc that attempts have been made to use it from accumulators in
places where a current supply is not available, but this cannot be
seriously recommended, except in special cases. Where an electric supply
is, however, available there can be no real choice, whether the lantern is
required for use in a large hall or a small class-room. The advantages of
using the arc are so great that no other method need be seriously
considered.
The one real objection that I have heard urged against it is due, curiously
enough, to its very perfection, and that is, that it lends itself to such
exceedingly sharp definition that any slight imperfection in the slide is
too faithfully reproduced on the screen, for which reason it is sometimes
recommended that the operator shall work with the objective the least
fraction out of focus; but this is a matter for individual taste and
judgment.
If, however, there is no possibility of using the electric current, one of
the other sources of illumination must perforce be adopted, and for a
_large_ hall this can only be limelight in one of its many forms, viz.
oxyhydrogen, oxyether, oxyacetylene, &c. As regards results on the screen,
this light compares well even against the electric arc, but it involves the
expense and trouble of compressed gas cylinders, or the infinitely worse
recourse to the now obsolete method of filling gas-bags.
Limelight is therefore now but little used in this country, as the majority
of large halls are equipped with the electric {5} current, and for smaller
buildings it is deemed unnecessary and too expensive.
ACETYLENE is undoubtedly the illuminant most in favour next to electric
light, as the light is brilliant enough to illuminate a picture 12 feet in
diameter at a distance up to, say, 30 feet from the screen, and this
suffices in a large majority of cases, and acetylene is comparatively
cheap, and reasonably simple to work.
INCANDESCENT-GAS is often employed for small class-rooms and is fairly
effective for a picture not exceeding 9 or 10 feet in diameter, and the
same can be said of the same type of burner heated by methylated spirit.
PARAFFIN-OIL is the poorest of all present-day forms of lantern
illuminants. The flame is large, impairing the definition, yellow in
colour, uneven in illumination, liable to smoke and smell, and barely equal
to incandescent gas in illuminating power.
It is therefore going gradually out of use in this country, but in
out-of-the-way places, especially abroad, it is sometimes the only
practicable light, and is therefore still employed from the best of all
reasons, necessity.
It is not the intention of the author to give precise working instruction
for all and every variety of the above illuminants as manufactured by
different firms. For such the reader must be referred to the directions
usually issued by the makers themselves, but a general description of the
various types offered for choice will not be out of place, and it will be
more convenient to begin with the poorest, viz. paraffin-oil, and finish
with the most perfect, the electric arc.
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