passing between the optophone and the light. A hand moved across
in front of it would give a purring sound, and a glimpse out of a
window in daylight would sound like a cinematograph reeling off a
film.

"Ah, there he is." Craig was listening with intense excitement
now. "Our intruder has entered the art-gallery. He is flashing his
electric light cane about at various objects, reconnoitring. No
doubt if I were expert enough and had had time to study it, I
could tell you by the sound just what he is looking at."

"Craig," I interrupted, this time very excited myself, "the
buzzing from the high-frequency current is getting lower and
lower."

"By George, then, there is another of them," he replied. "I'm not
surprised. Keep a sharp watch. Tell me the moment the buzzing
increases again."

Spencer could scarcely control his impatience. It had been a long
time since he had been a mere spectator, and he did not seem to
relish being held in check by anybody.

"Now that you are sure the vandal is there," he cut in, his cigar
out in his excitement, "can't we make a dash over there and get
him before he has a chance to do any more damage? He might be
destroying thousands of dollars' worth of stuff while we are
waiting here."

"And he could destroy the whole collection, building and all,