In 1885, while seeking means to indicate readily the intensities of vocal sounds, I first met with these [voice] figures, and, owing to their variety both in form and production, they have since absorbed much of my attention. The apparatus I have employed I call the eidophone. This is very simple. It consists merely of an elastic membrane, such as thoroughly flexible soft sheet-rubber, tightly stretched over the mouth of a receiver of any form, into which receiver the voice is introduced by a wide-mouthed tube of convenient shape. In some cases the receiver may be dispensed with, and the membrane be stretched across the open end of the tube itself.

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My first experiments were made with sand, lycopodium powder, or the two substances mixed. I then tried for the production of voice-figures, flooding the disk of the eidophone with a thin layer of liquid ; e.g. water or milk. Upon singing notes of suitable pitch through the tube, not too forcibly, beautiful crispations appear upon the surface of the liquid, which vary with every change of tone. A note sung too forcibly causes the liquid to rise in, a shower of spray, the movements of which are too rapid to be readily followed by the eye. To facilitate observation denser liquids may be used. By using such liquids as colored glycerin particularly beautiful effects may be obtained. Subsequently I found that by employing moistened powder of different consistencies yet another description of figures appears. The earliest result of my experiments in this material shows centers of motion from which radiations diverge.