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was stated in the last chapter that however light the contact may be, and however smooth the glass recording-surface, the record was still apt to be either arrested, or seriously distorted, on account of friction. As long as I employed the ordinary method of continuous contact of the writing-point with the glass surface, it was impossible to overcome this particular difficulty. It occurred to me at last that the problem might find a solution if I could succeed in making an intermittent instead of a continuous writing-contact. I have solved this problem by devising two different types of apparatus, which I have called respectively the Oscillating Recorder and the Resonant Recorder. In the former, the recording-surface itself is made by an electro-magnetic device, to vibrate to and fro, thus bringing it into periodic contact with the writing-point. This apparatus is extremely convenient for the general purpose of recording responses in which the measurement of excessively short intervals of time is not essential.

But there are many important problems, such as the determination of the latent period, and accurate determination of the velocity of transmission of excitation, in which