HOW IT WORKS.

very minute indentations, not exceeding 1⁄1000th of an inch in depth.³³ Seen under a microscope, the surface of the record is a succession of hills and valleys, some much larger than others (Fig. 151, a). A loud sound causes the stylus to give a vigorous dig, while low sounds scarcely move it at all. The wonderful thing about this sound-recording is, that not only are the fundamental tones of musical notes impressed, but also the