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word how much I feel indebted to him for the great interest he has taken in these lectures. This instrument has a resistance of no less than 86,000 ohms, and it is one of the most sensitive of its kind. Fig. 68.—Mirror and ring-magnet of Wiedemannn's galvanometer. B, mirror; A, ring-magnet. Now if I brought the wires of this instrument into direct contact with the muscle, we would probably get a current; but that would be no proof that the current really came from the muscle. The copper wires come into contact with the moist surface of the muscle, and this contact would at once, by chemical action, generate a current of electricity. For example, to show you how easily currents can be produced and detected by an instrument of this kind, observe that when I touch the wires the spot of light at once moves. We must have some means, therefore, of leading off from the muscle any