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 



