With the optical method of making the vibrations of a tuning fork visible, a beam of light coming from an intensely illuminated pinhole is sent to a mirror fixed to a prong and the reflected beam is seen on a screen or observed directly by the eye.
When the tuning fork is immobile, the motionless image of the pinhole shows a dot of light; when the tuning fork is vibrating the mirror oscillates around the axis perpendicular to the plane formed by the two prongs, the image moves parallel to that plane and, owing to the persistence of the image on the retina, since the movement is quite rapid, a persistent rectilinear image is seen.
To study the composition of two parallel vibrating movements with this method, two tuning forks arranged vertically, one in front of the other, are employed. In this way the beam of light reflected by the mirror of one of them is then reflected by the mirror of the other, which is parallel to the former, on the screen placed on the back focal plane of a convergent lens.
When only one of the two tuning forks is made to vibrate, a line is observed; if the two forks are made to vibrate with the same frequency, the line becomes longer or shorter depending on the difference in phase; the line is at its longest when the phase is the same and at its shortest when the phases are in opposition.
If we shift the cursors on the prongs slightly, we vary the frequency of the second fork, thus producing beats, and the line lengthens and shortens periodically. ...continues