Instead of a lime is used a spherical 'Pastille' of peculiar composition, and before use each pastille must be burnt off exactly like an incandescent gas mantle, after which it is extremely fragile and difficult to handle.
To use this illuminant one lens of the condenser must be removed, the curvature of the mirror taking its place, and it will be seen at once that the pastille itself will get in its own
light and throw a shadow, which actually happens, but it is hardly perceptible unless specially looked for.
A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve, pressure gauge and burner, with two spare pastilles, is shown in Fig. 24, but if preferred a regulator, such as previously described for oxygen, can be used instead of the fine adjustment valve.
Fallot Oxy-Acetylene Blast.—This is similar to the foregoing, utilising oxygen from a cylinder instead of air, and the light is equal to a powerful limelight, and may be considered as an efficient substitute, though for long range work the shadow before alluded to becomes more noticeable (for optical reasons which need not be here discussed). The Fallot Company also make a special 'Pressure Generator' which can be used instead of a D.A. Cylinder; but my experience of this so far is that, although perfectly safe, the blast from it is a little unsteady as compared with a cylinder.
Limes and Accessories.—Limes for Optical Lantern work are usually supplied in the form of cylinders, the 'ordinary' size being ⅞ inch in diameter and about 1½ inches long, with a hole drilled longitudinally to take the lime pin. Extra large limes up to 2 inches in diameter are supplied for more powerful jets.
So-called 'soft' limes used to be recommended for 'blow-through' jets as giving a better light than 'hard' limes, but the advantage, if any, is very little, and these limes are now very seldom heard of, possibly because 'blow-through' jets themselves are becoming less and less used, and 'soft' limes will not stand the heat of a mixed or 'Injector' jet for long.