Page:Elementary Text-book of Physics (Anthony, 1897).djvu/194

180 to the rear of the capsule, and singing into it the different vowel sounds, the flame images assume distinct forms for each. Some of these forms are shown in Fig. 60.

154. Optical Method of Studying Vibrations.—The vibratory motion of sounding bodies may sometimes be studied to advantage

by observing the lines traced by luminous points upon the vibrating body or by observing the movement of a beam of light reflected from a mirror attached to the body.

Young studied the vibrations of strings by placing the string where a thin sheet of light would fall across it, so as to illuminate a single point. "When the string was caused to vibrate, the path of the point appeared as a continuous line, in consequence of the persistence of vision. Some of the results which he obtained are given in Fig. 61, taken from Tyndall on Sound.

The most interesting application of this method was made by Lissajous to illustrate the composition of vibratory motions at right angles to each other. If a beam of light be reflected to a screen from a mirror attached to a tuning-fork, when the tuning-fork vibrates the spot on the screen will describe a simple harmonic motion and will appear as a straight line of light. If the beam, instead of being reflected to a screen, fall upon a mirror attached to a second fork, mounted so as to vibrate in a plane at right angles to the first, the spot of light will, when both forks vibrate, be actuated by two simple harmonic motions at right angles to each other, and the resultant path will appear as a curve more or less complicated, depending upon the relation of the two forks to each other as to both period and phase (§ 21). Fig. 62 shows some of