After obtaining the balance, if we apply an exciting reagent like Na_2CO_3 at one point, and a depressing reagent like KBr at the other, the responses will now become unequal, the more excitable point giving a stronger deflection. We can, however, make the two deflections equal by increasing the amplitude of vibration of the less sensitive point. The two deflections may thus be rendered equal and opposite, but the time relations--the latent period, the time rate for attaining the maximum excitation and recovery from that effect--will no longer be the same in the two cases. There would therefore be no continuous balance, and we obtain instead a very interesting diphasic record. I give below an exact reproduction of the response-curves of A and B recorded on a fast-moving drum. It will be remembered that one point was touched with Na_2CO_3 and the other with KBr. By suitably increasing the amplitude of vibration of the less sensitive, the two deflections were rendered approximately equal. The records of A and B were at first taken separately (fig. 66, _a_). It will be noticed that the maximum deflection of A was attained relatively much earlier than that of B. The resultant curve R' was obtained by summation.
[Illustration: FIG. 66.--DIPHASIC VARIATION (_a_) Records of A and B obtained separately. R' is the resultant by algebraical summation. (_b_) Diphasic record obtained by simultaneous stimulation of A and B.]
After taking the records of A and B separately, a record of resultant effect R due to simultaneous vibration of A and B was next taken. It gave the curious two-phased response--positive effect followed by negative after-vibration, practically similar to the resultant curve R' (fig. 66, _b_).
The positive portion of the curve is due to A effect and the negative to B. If by any means, say by either increasing the amplitude of vibration of A or increasing its sensitiveness, the response of A is very greatly enhanced, then the positive effect would be predominant and the negative effect would become inconspicuous. When the two constituent responses are of the same order of magnitude, we shall have a positive response followed by a negative after-vibration; the first twitch will belong to the one which responds earlier. If the response of A is very much reduced, then the positive effect will be reduced to a mere twitch and the negative effect will become predominant.