He must respond to it.

The duty lay with him; he must answer this black infidel for the Church, for faith, for God.
The more he thought of it, the more Charley's face came before him, with the monocle shining and hard in the eye.

The monocle haunted him.

That was the infidel's sign.

"Show me a sign from Heaven, tailor-man!" What sign should he show?
Presently he sat up straight in bed.