Lissajou's Figures. If rays of light are allowed to fall on a mirror fixed to one branch of a tuning fork they will be reflected, and we can place a second mirror (similarly attached to another tuning fork) so that the rays reflected from the first fall upon the second, and are again reflected on to a screen. We can thus obtain a spot of light on the screen, and by letting either fork vibrate alone, the motion of its attached mirror will cause the spot to travel to and fro in a straight line on the screen. On account of the persistence of vision we then see a line of light. If the first fork can vibrate in a vertical plane and the second in a horizontal one, both being at right angles to the screen, then the line of light will be either vertical or horizontal according as the first or second fork vibrates alone. If the forks vibrate together the spot of light describes a curve, and the curves so obtained are known as Lissajou's figures. They are of countless different shapes, being determined by the rates at which the forks vibrate. The experiment is an extremely beautiful one, and is also of practical importance, since it gives a means for comparing different tuning forks.