be disregarded, but if the entire record is to be measured it is a source of error.  Hensen[16] first turned the phonautograph to account for the study of speech.  He used a diaphragm of goldbeater’s skin, of conical shape, with a stylus acting over a fulcrum and writing on a thinly smoked glass plate.  The apparatus was later improved by Pipping, who used a diamond in place of the steel point.  The diamond scratched the record directly on the glass.  The Hensen-Pipping apparatus has the advantage of taking records directly in the plane of the surface, but it does not make a record which can be reproduced; in case of doubt as to the exact thing represented by the curve, there is no means of referring to the original sounds; and it involves working with a microscope.

   [16] Hensen:  Hermann’s Handbuch d.  Physiol., 1879, Bd.  I., Th. 
   II., S. 187.

[Illustration:  FIG. 3.  Diagrammatic section of recording apparatus. a, diaphragm; s, stylus; g, guide; p, section of plate.]

The apparatus which was used in the following experiments consisted essentially of two recording devices—­an ordinary phonograph, and a recorder of the Hensen type writing on a rotary glass disc (see Fig. 5, Plate X.).  Of the phonograph nothing need be said.  The Hensen recorder, seen in cross section in Fig. 3, was of the simplest type.  A diaphragm box of the sort formerly used in the phonograph was modified for the purpose.  The diaphragm was of glass, thin rubber, or goldbeater’s skin.  The stylus was attached perpendicularly to the surface of the diaphragm at its center.  The stylus consisted of a piece of light brass wire bent into a right angle; the longer arm was perpendicular to the diaphragm; the shorter arm was tipped with a very fine steel point, which pointed downward and wrote on the disc; the point was inclined a trifle to the disc, in order that it might ‘trail,’ and write smoothly on the moving disc.  The stylus had no fulcrum or joint, but recorded directly the vibrations of the diaphragm.  In early experiments, the diaphragm and stylus were used without any other attachment.

But a flexible point writing on smoked glass is a source of error.  When the disc revolves under the stylus, the flexibility of the diaphragm and of the stylus permit it to be dragged forward slightly by the friction of the moving surface.  When the diaphragm is set vibrating the conditions are altered, and the stylus springs back to nearly its original position.  The apparent effect is an elongation of the earlier part of the curve written, and a corresponding compression of the last verse written.  This error is easily tested by starting the disc, and without vibrating the diaphragm stopping the disc; the stylus is now in its forward position; speak into the apparatus and vibrate the diaphragm, and the stylus will run backward to its original position, giving an effect in the line like a (Fig. 4).  If the error is eliminated, the stylus will remain in position throughout, and the trial record will give a sharp line across the track of the stylus as in b.