Arthur R. Laufer, author of the article on “Ultrasonics” which appeared in the August issue of Physics Today, has called our attention to an error in the caption at the bottom of page 12 of that issue. The photograph shows the focusing of an ultrasonic beam by a Lucite lens. Since the velocity of ultrasonic waves is greater in Lucite than it is in water or other liquids, it is necessary to use a negative lens in order to produce convergence of the beam. The lens shown in the photograph is a plano‐concave lens and not a plano‐convex lens as stated in the legend.

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