We understand; the trick 's but natural:
Such spirits' individuality
Is hard to put in evidence: they incline
To gibe and jeer, these undeveloped sorts.
You see, their world 's much like a jail broke loose,
While this of ours remains shut, bolted, barred,
With a single window to it. Sludge, our friend,
Serves as this window, whether thin or thick,
Or stained or stainless; he 's the medium-pane
Through which, to see us and be seen, they peep: