Visualizations of vibration patterns from 1787 by Ernst Chladni:
One of Chladni’s best-known achievements was inventing a technique to show the various modes of vibration of a rigid surface. First published in 1787 in his book Entdeckungen über die Theorie des Klanges, the technique consists of drawing a bow over a (circular, square, or rectangular) plate or membrane whose surface is lightly covered with sand. When stroked, a given plate will resonate at one of its natural frequencies. The sand bounces about on the plate until settling at nodal points (areas of zero movement) thereby producing intricate patterns. These patterns are now called Chladni figures.