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fo4 OPTICAL PROJECTION moonlight scene ; or a ship, to the same ship on fire ; or summer to winter. These effects retain all their charm, and many sets of such can be procured; but dissolving one slide into quite a different one has no meaning, and, indeed, needs constant care to avoid now and then absolutely ludicrous sequences. 57. Registration of Slides.—The perfection of dissolving depends upon absolutely perfect registration, as it is called. That is, every line of the picture must dissolve precisely into its corresponding line of the other slide, and in more ordinary changes, at least each disc should precisely correspond with the preceding. To ensure this, in the first place the two objectives must be of precisely the same magnifying power; and this should not be taken for granted without actual proof, which is best afforded by two duplicate plain diagrams, or slides of printed matter. In the second place the two lanterns must be carefully adjusted that their discs may coincide upon the screen. And besides all this, the slides themselves must be carefully ' registered.' A hired set of slides can never be shown in perfect registration for want of this last, which is impossible with such a ' scratch' set. The best lanterns are, and should be, fitted with stages which have adjustable screw stops, by which the precise places of the two discs can be adjusted. If this is carefully done, most slides will register fairly as regards their discs, and this degree of registration satisfies what we may call the average exhibitor. Further even than this, the mere discs may be absolutely registered by a plan sometimes adopted of exhibiting the slides (packed or carried loose) in a frame or carrier with an aperture slightly less than the slide itself shows. This slight contraction of the visible surface is sufficient to cover any little inaccuracy in the slides. Another method is, for the operator to watch carefully the disc of the new slide whilst the light is only just suffi- ciently turned on to show it almost imperceptibly, and then