I still haven't finished Pale Fire. I've stopped reading it because I don't want it to be over. My favorite part so far is John Shade's daughter's encounter with the poltergeist, a spot of light that she tries to speak with by reciting the alphabet and recording which letters it bounces on. The poltergeist reminds me of those blind spots you get in your eyes when a camera flash goes off. And in fact orbs appearing on digital photos are supposedly evidence of poltergeists in people's houses.
The flash-spot moves around with your vision, which drives you crazy, like the belief that your house is haunted. All you can see is that you can't see what's there. The orb of light in Pale Fire seems like a version of the Nedotykomka in Sologub's Petty Demon. The Nedotykomka is a sort of malevolent dust bunny that represents and drives the protagonist's madness. I now see that David Bethea in Russian Review (63:1) has found reflections of the Petty Demon in Nabokov, so I'm not the only one. It might be that the orb is not just a repercussion of the Nedotykomka, but an image of unacknowledged literary influence--a hole burned in the page. Influence is definitely maddening.