with an electric current-regulator, the essential principle of the electric current-regulator being the employment of a loose or imperfect contact between two parts of the conducting system, so arranged that the vibrations of the tympanum would alter the degree of contact and thereby interrupt in a corresponding degree the passage of the current. [Illustration: Fig. 6.] Mr. Horkheimer, a former pupil of Reis, informs me that a much larger model of the ear was also constructed by Reis. No trace of this is, however, known. _Second Form._--TIN TUBE. The second form, a tube constructed by Reis himself, of tin, is still to be seen in the Physical cabinet of Garnier’s Institute, at Friedrichsdorf, and is shown in Fig. 7. It consists of an auditory tube _a_, with an embouchure representing the pinna or flap of the ear. This second apparatus shows also a great similarity with the arrangement of the ear, having the pinna or ear-flap, the auditory passage, and the drum-skin (_a_, _b_, _c_). Upon the bladder _c_ there still remains some sealing-wax, by means of which a little strip of platinum, for the all-essential loose-contact that controlled the current, had formerly been cemented to the apparatus. [Illustration: Fig. 7.] _Third Form._--THE COLLAR-BOX. [Illustration: Fig. 8.] The third form, also preserved in the collection in Garnier’s Institute, is given in Fig. 8, which, with the preceding, is taken by permission from the pamphlet of the late Professor Schenk, consists of a round tin box, the upper part of which fits upon the lower precisely like the lid of a collar-box. Over this lid _b_, which is 15 centimetres in diameter, was formerly stretched the vibrating membrane, there being also an inner flange of metal. Into a circular aperture below opened an auditory tube _a_, with an embouchure representing the pinna. The precise arrangements of the contact-parts of this apparatus are not known. Mr. Horkheimer, who aided Reis in his earlier experiments, has no knowledge of this form, which he thinks was made later than June 1862. This is not improbable, as the design with horizontal membrane more nearly approaches that of the tenth form, the “Square-box” pattern. _Fourth Form._--THE BORED-BLOCK. The instrument described by Reis in his paper “On Telephony,” in the Annual Report of the Physical Society of Frankfort-on-the-Main, for 1860-61 (see p. 50), comes next in order. The inventor’s own description of this telephone (Fig. 9) is as follows:-- [Illustration: Fig. 9.]