At this point, going by this, humankind's knowledge of the supernatural is really primitive. Mr. Asama used focusing of lenses as an analogy... well, it doesn't look like you can get prescription "ghost glasses", from a "ghost-eye doctor" (see note), if your "ghost vision" sucks. Sucks to be you if you need that, just like how it would have sucked to be someone with bad regular ol' eyesight in 200 B.C.
Note: Not to be confused with "ghost eye-doctor".