The photographs of sound waves obtained with the phonodeik have wave lengths varying from 25 to 100 millimeters; these waves are enlarged with the apparatus shown in Fig. 85. The photographic film negative of the wave is placed in an adjustable holder f, on an optical-bench projection lantern, the curve being projected on a movable easel e; adjustments are made until the projected wave is of the proper size, is well defined, and has its axis horizontal; the curve is then traced with a pencil on a sheet of paper.

The initial point is chosen with reference to convenience in determining the length of one wave. The time required for the operation of enlarging a curve is less than five minutes.

Thus all curves as analyzed are of the same wave length, regardless of their original size and frequency, and as they are drawn on a standard sheet of paper, 19 by 24 inches, filing is facilitated. The harmonic synthesizer, described later, draws curves of this same wave length, 400 millimeters, which permits a direct comparison of the analyzed and synthesized curves.