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this Institution. Professor Dewar has kindly Fig. 67.—Wiedemann's galvanometer, much employed, especially in Continental schools, by physiologists. The instrument used in the lecture was a Thomson's (Lord Kelvin) galvanometer, and it is shown in Fig. 58, b. Wiedemann's instrument I have often used with great advantage for class demonstration. The two outer coils are of low, and the two inner coils are of high, resistance. The low resistance coils are used for thermal currents, and the high resistance coils for the currents of living tissues. A ring magnet is suspended by a long filament of silk, and hangs in a copper box in the centre of the coils of wire. On the rod carrying the ring-magnet, and in the box above the coils, we have a mirror, which reflects a beam of light on to the scale. placed it at my service, and I wish to say in a