In use, the box holding the fixed reed would be rotated
90 degrees, making it oscillate at right angles to the other reeds. A beam of light reflecting from the two reeds in succession undergoes simple harmonic motion in two perpendicular directions, thus producing a Lissajous figure when the beam strikes a screen. The shape of the figures produced by two frequencies which bear small number ratios to each other is well-known.

   The apparatus is made by Newton and Company of London, and dates from the last quarter of the nineteenth century. It is in the collection of the National Museum of American History of the Smithsonian Institution.