Light is shown through two oscillating plates. One has a vertical slit and oscillates horizontally. The other has a horizontal slit and oscillated vertically. Either slit can be set oscillating alone to show the two components of the final form when bo th are set in motion. The period can be adjusted on the one with the horizontal slit -- a stationary figure results with an integral ratio of periods. Shine light through the slits with the vertical slit, square aperture toward light, nearest the light s ource. Use a lens to project the image onto a screen.