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I played a game or two of Myst long ago, but I don't know what visual effect you're trying to describe.
In advance: I think what you're describing will likely end up requiring Flash in addition to Javascript, or else a whole lot of gif images plus Javascript.
As for the object itself, not sure how I'd approach that, it can be an actual object you move via Javascript or easier yet, a Flash object that accepts input from the Javascript function.
Look up some "scrolling status bar" scripts, it's a similar concept.