Page:Collected Physical Papers.djvu/317

Rh is more pronounced when feeble stimulation gives rise, as to be presently explained, to a response of opposite sign to that of the normal. The curve is straight in the middle and concave in the last part where the amplitude of response reaches a limit.

Effect of Sub-minimal Stimulus.—The response is of opposite sign to that of the normal, when the intensity of stimulus is sub-minimal. This characteristic effect I also find in the response of living tissues.

If instead of a single vibration of a given amplitude we superpose a rapidly succeeding series, the individual effects are added-up and a maximum deflection is produced which remains practically constant as long as the vibration is maintained. A single ineffective stimulus thus becomes effective by the additive effect of several. Too long-continued vibration may cause fatigue, but during half a minute or so, the maximum effect in tin is very definite. For example, a single vibration of 5° gave a deflection of 3.5 divisions; the same when continued at the rate of four times per second gave a maximum deflection of eighteen divisions. A single vibration of 10° of the same wire gave a deflection of 4.5 divisions, but continued vibration gave the definite maximum of 37.5 divisions. I give below a curve (fig. 80) exhibiting the maximum effects for different amplitudes of vibration.

Hysteresis.—Allusion has already been made as to the increased sensitiveness conferred by preliminary vibration. Being desirous of finding out in what manner this is brought about, I took a series of observations for an entire cycle, that is to say, a series of observations