A veil hangs in between mankind and that which is hidden.  It was put there for a reason.  It can be removed rightfully by only God Himself, and not by otherworldly or occult means (Luke 23:45).  He created this separation just as He separated the waters above from the waters below by the sky in which fallen spirits now rule (Ephesians 2:2).  Sometimes these spirits are visible, and sometimes they’re visible in odd or obscure ways.  They may at times appear to phase into and out of the physical world, just as moving the dial on an analog radio may at times pick up two signals at once or nothing but noise.  Moreover, their radiance may vary depending on the grace associated with them or, using the radio analogy, the strength of their signal.  However, one should never extend an invitation to these spirits of the air.  They've fallen from their original domain, and it would be foolish to invite them into yours.