A ghostglass is a cloudy but mostly transparent quartz lens the size of a human palm. Both sides are convex. The uneven, smooth edges suggest careful but inexpert carving a very long time ago.
"Hello in there."
-Harrier Wren to the demon Xiahael the Unforgivable, while peering into his host's eye with a ghostglass
The cloudy view through a ghostglass reveals the presence of spirits and other incorporeal beings, though not in any great level of detail. Up close, one might get a look at their faces or a better sense of their natures or desires. A view of a haunted house might indicate the general hauntedness of said house, maybe centred on a specific room.
As people are constantly dying in
cities, a good deal of wispy visual clutter makes the ghostglass view even foggier than normal inside any given city.
Each time one uses a ghostglass, one becomes a little more perceptible, interesting, and/or appetizing to incorporeal beings. The necromancer
Harrier Wren, who carries a ghostglass habitually, has suffered many ill effects from this cost (though of course she's also turned it to her advantage more than once).
A ghostglass held up against a mirror can allow self-examination of the 'oh dear I appear to be possessed' variety.