'Hard' limes are turned out of the hardest stone lime, and must be kept in sealed tins until used, as they rapidly disintegrate when exposed to the air. There are one or two quarries known to provide the best lime for lantern purposes, and the various good brands on the market practically have the same origin as regards raw material, though called by different trade names; and the 'Hardazion' (hard as iron) limes, placed on the market some years ago by a well-known wholesale firm, to be countered shortly after by the 'Hardastil' (harder still) brand, are, I take it, legitimate though amusing instances of phonetic advertisement.
Even the best of limes is liable to crack under the heat
of a powerful jet, and so a pair of lime-tongs should always be provided, and there is nothing better than the simple form shown in Fig. 25, and which is, or should be, sold by all dealers.
Substitutes for Limes.—A good substitute for lime, that will give the same light, stand heat equally as well, and not deteriorate if exposed to the atmosphere, has long been sought for, and some of the more recently discovered refractory materials are more or less satisfactory. 'Mabor' limes, for example, belong to this class, and so do some of the 'pastilles,' which before the war came chiefly from France and to a less extent from Germany.
CHAPTER VI
THE ELECTRIC LIGHT
The electric current provides the light for an optical lantern, though it may take various forms, such as the incandescent glow-lamp in some shape or other, the comparatively new Ediswan 'Pointolite' lamp, the enclosed arc, and the open arc. This little book is not a treatise on electricity, but a few elementary notes may not be out of place, and may be of assistance to the non-technical lanternist.
The first point then to be considered in adopting the electric light for the purpose of lantern projection is the character of the supply, and the information required may be summed up thus: (1) E.M.F., voltage, or tension (these three expressions having exactly the same meaning); (2) ampèrage or amount of current available; (3) whether current is (a) continuous, constant, or direct (again three words meaning