The use of lenses to focus waves other than those of light is not new, either in the field of acoustics or that of radio. Indeed, many physics students must have seen demonstrations of the bending of sound waves by a prism of carbon dioxide confined within a membrane. Hertz himself used prisms of pitch to bend the short radio waves with which he worked in demonstrating the like nature of light and of the electromagnetic radiation predicted by Maxwell. These were laboratory experiments, however, and it was not until lenses were used in connection with microwave antennas during and after the war that the full potentialities of the dioptrics of microwaves, and later, of sound, became apparent.

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