I give a series of records, fig. 67, in which these three principal types are well exhibited, the two contacts having been rendered unequally excitable by solutions of the two reagents KBr and Na_2CO_3. A and B were vibrated simultaneously and records taken. (_a_) First, the relative response of B (downward) is increased by increasing its amplitude of vibration. The amplitude of vibration of A was throughout maintained constant. The negative or downward response is now very conspicuous, there being only a mere preliminary indication of the positive effect. (_b_) The amplitude of vibration of B is now slightly reduced, and we obtain the diphasic effect. (_c_) The intensity of vibration of B is diminished still further, and the negative effect is seen reduced to a slight downward after-vibration, the positive up-curve being now very prominent (fig. 67).
[Illustration: FIG. 67.--NEGATIVE, DIPHASIC, AND POSITIVE RESULTANT RESPONSE]
#Continuous transformation from negative to positive.#--I have shown the three phases of transformation, the intensity of one of the constituent responses being varied by altering the intensity of disturbance.
In the following record (fig. 68) I succeeded in obtaining a continuous transformation from positive to negative phase by a continuous change in the relative sensitiveness of the two contacts.
I found that traces of after-effect due to the application of Na_2CO_3 remain for a time. If the reagent is previously applied to an area and the traces of the carbonate then washed off, the increased sensitiveness conferred disappears gradually. Again, if we apply Na_2CO_3 solution to a fresh point, the sensitiveness gradually increases. There is another further interesting point to be noticed: the beginning of response is earlier when the application of Na_2CO_3 is fresh.
[Illustration: FIG. 68.--CONTINUOUS TRANSFORMATION FROM NEGATIVE TO POSITIVE THROUGH INTERMEDIATE DIPHASIC RESPONSE Thick dots represent the times of application of successive stimuli.]
We have thus a wire held at one end, and successive uniform vibrations at intervals of one minute imparted to the wire as a whole, by means of a vibration head on the other end.
Owing to the after-effect of previous application of Na_2CO_3 the sensitiveness of B is at the beginning great, hence the three resultant responses at the beginning are negative or downward.