I acquiesced.
"This is an instrument which literally makes light audible," he pursued.
"Hear light?" I repeated, in amazement.
"Exactly," he reiterated. "You've said it. It was invented to assist the blind, but I think I'll be able to show that it can be used to assist justice—which is blind sometimes, they say. It is the optophone."
He paused to adjust the thing more accurately and I looked at it with an added respect.
"It was invented," he resumed, "by Professor Fournier d'Albe, a lecturer on physics at the University of Birmingham, England, and has been shown before many learned societies over there."
"You mean it enables the blind to see by hearing?" I asked.
"That's it," he nodded. "It actually enables the blind to locate many things, purely by the light reflected by them. Its action is based on the peculiar property of selenium, which, you probably know, changes its electrical conductivity under the influence of light. Selenium in the dark is a poor conductor of electricity; in the light it, strange to say, becomes a good conductor. Variations of light can thus be transmuted into variations of sound. That pushed-in end of the box which we hid over in Warrington's had, as you might have noticed, a selenium plate on the inside partition, facing the open end of the box."
"I understand," I agreed, vaguely.
"Now," he went on, "this property of selenium is used for producing or rather allowing to be transmitted an electric current which is interrupted by a special clockwork interrupter, and so is made audible in this wireless telephone receiver which I have here connected with this second box. The eye is replaced by the ear as the detector of light—that is all."