The vibration microscope is an electromagnetically-driven adaptation by Hermann von Helmholtz (1821-1894) of the earlier optical comparator by Jules Antoine Lissajous (1822-1880). The device allows one to determine the frequency of a tuning fork or other vibrating object with respect to a fork of known frequency, by way of Lissajous figure analysis.
Wooden sticks, when dropped on the floor, sound a variety of tones. While the bars of a xylophone are varied in tone by changing their length, these “tone bars” are all of the same length and width, but have different thicknesses and different densities and elastic properties.
Helmholtz described in his 1862 [sic] book, On the Sensations of Tone, an apparatus able to pick out specific frequencies from a sound. The Helmholtz resonator, as it is now called, consists of a rigid container of a known volume, nearly spherical in shape, with a small neck and hole in one end and a larger hole in the other end to admit the sound.