CHAPTER II
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
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
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.