Earlier investigations in this field have left the factors of intensity, rate of transmission (and consequently time of stimulation) and phase without adequate control. In this investigation, as far as it has proceeded, we have attempted to supply this deficiency.
In the experimenter's room is a speaker that may be moved right or left upon the arc of a circle. Before it, in fixed positions, are two microphones. These microphones communicate with two vibrators, respectively, in the distant observer's room. The observer is seated with the index finger of the right hand upon one vibrator and that of the left hand upon the other. The situation is so arranged that he cannot hear the action of the vibrators. Before him, and covering the vibrators, is a sheet of cardboard on which the arc of a semicircle is described marked to designate degrees.
As the speaker, in action, is moved sufficiently to the right or left, thus varying its distance from the microphones, respectively, the intensity, time and phase of the stimulus upon the observer's fingers, respectively, are altered as in a corresponding situation in the sphere of audition. The observer “feels as if” a vibrating body before him were moving between right and left. On the semi‐circumference before him he designates the degree of the apparent movement.
When, now, the speaker is kept in a constant middle position with reference to the microphones and when attenuators are introduced to control intensity in the vibrators, respectively, the time element is kept constant but the observer continues to experience the illusion of movement referred to. The degree of apparent movement corresponds to that that obtains in the auditory sphere in similar situations.
We have yet to complete the experiment by holding intensity constant while other factors are independently varied.