This lantern is called a "biunial" (two in one) it was a precision instrument used to create startling special effects on the screen. With such an apparatus one slide could be made to dissolve into another creating startling effects (e.g. day becoming night etc). Even more complex results could be achieved with a "triunial" ( three in one lantern)
Above a matching pair of dissolving view effect slides.
Dissolving views could be used to make one picture subtly change into another or superimpose one image on top of another to add drama to a show (for example the appearance of angels )
Press the red button below to see the dissolving view effect