Then I moved it until the purring sound became as faint as
possible. The instrument is at the present moment in its most
sensitive state."
"What does it sound like?" I asked.
"Well, the passage of a hand or other object across the aperture
is indicated by a sort of murmuring sound," he replied, "the
loudest sound indicating the passage of the edges where the
contrast is greatest. In a fairly bright light, even the swiftest
shadow is discoverable. Prolonged exposure, however, blinds the
optophone, just as it blinds the eye."
"Do you hear anything now?" I asked watching his face curiously.
"No. When I turned the current on at first I heard a ticking or
rasping sound. I silenced that. But any change in the amount of
light in that dark room over there would restore the sound, and
its intensity would indicate the power of the light."
He continued to listen.
"When I first tried this, I found that a glimpse out of the window
in daylight sounded like a cinematograph reeling off a film.
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