The first experiment which I have to describe relates to an attempt to employ the telephone as a means of measuring the hearing power of the ear in different individuals, or in the same individual at different times. For this purpose a constant source of sound, driven by clockwork, is made to act upon the disc of a telephone in a distant room. In the circuit of this telephone are included two others, so placed on a stand, that they can be brought close to both ears of the person to be experimented on. Soft india-rubber rings are attached to these telephones so as to enclose both ears, and Serve the double purpose of avoiding any disagreeable pressure on them, and preventing, as far as possible, all extraneous sounds from entering. In the circuit of the telephones there is also included a column of slightly acidulated water whose length can be varied at pleasure, so as to introduce a greater or less resistance to the electric current. This is managed in the following way:—A pretty wide glass tube, about 24 centimetres long, closed at the lower end, and graduated to millimetres, is mounted vertically on a stand. Projecting through the lower end of the tube is a platinum wire melted into the glass and having its point inside opposite the zero of the scale.