pistol where he left them. Now, Walter, since you have become
accustomed to this thing take it and tell me what you hear."

Craig had already seized the other apparatus connected with the
art-gallery and had the wireless receiver over his head. He was
listening with rapt attention, talking while he waited.

"This is an apparatus," he was saying, "that was devised by Dr.
Fournier d'Albe, lecturer on physics at Birmingham University, to
aid the blind. It is known as the optophone. What I am literally
doing now is to HEAR light. The optophone translates light into
sound by means of that wonderful little element, selenium, which
in darkness is a poor conductor of electricity, but in light is a
good conductor. This property is used in the optophone in
transmitting an electric current which is interrupted by a special
clockwork interrupter. It makes light and darkness audible in the
telephone. This thing over my head is like a wireless telephone
receiver, capable of detecting a current of even a quarter of a
microampere."

We were all waiting expectantly for Craig to speak. Evidently the
intruder was now mounting the stairs to the art-gallery.

"Actually I can hear the light of the stars shining in through
that wonderful plate glass skylight of yours, Mr. Spencer," he
went on. "A few moments ago when the moon shone through I could
hear it, like the rumble of a passing cart. I knew it was the moon
both because I could see that it must be shining in and because I
recognised the sound. The sun would thunder like a passing
express-train if it were daytime now. I can distinguish a shadow