Page:Proceedings of the Royal Society of London Vol 60.djvu/362

Rh Professor Hermann questions the accuracy of my method of analysis wlien appiind. to steep curves.

My answer is that I do not employ it in such cases, preferring to take photographs of sudden changes upon plates moving with sufficient rapidity to suitably develope the curves.

Thus in Professor Burdon Sanderson’s paper,* figs. 1, 2, 3, and 4, on Plate 1, and figs. 3, 4, and 5, on Plate 3, were intended to show within the limits of a page the entire course of certain phenomena. I did not analyse them, but simply measured the times of the maxima and minima. The remaining curves, viz., figs. 5 and 6, Plate 1, figs. 1_7 oxl Plate 2, and figs. 1 and 2 on Plate 3, are all suitable for analysis, with the exception of the first phase of fig. 1, which is almost too steep. I have done some thirty or forty of this kind.

As regards the further criticisms, so far as the physical interpretation of the curves is concerned, I can only say that cases did occur in which the maximum E.M.F. of the second (positive) phase exceeded the maximum E.M.F. of the first (negative) phase of the same response. "With respect to curves, like those in figs. 3 and 4, Plate II, the part referred to by Professor Burdon Sanderson as the “ hump,” is not merely the curve of discharge. The actual negatives which I measured show a rise of the meniscus after its rapid downward movement has ceased, and while it is still above the zero line, and a similar rise is plainly visible to the eye after every one of the “ spikes ” in figs. 1, 2, and 4, Plate 1, which were photographed with the machine moving more slowly.

It is impossible for the mercury, under these conditions, after approaching the zero line, to recede from, without crossing it, except under the influence of a negative Acting 1 .D. that is to say, the Impressed E.M.F. must be of the same sign as the charge already in the instrument, but must be of higher potential difference. In some negatives this second rise in followed by a descent more rapid than that of the curve of discharge, and therefore indicating a small positive Acting P.T). I first noticed and called attention to it in connection with the curves illustrating my paperf on the “ Time Relations of the Capillary Electrometer,” but refrained from discussing its physiological significance.

f ‘ Phil. Trans.,’ A, vol. 183, p. 104.
 * ‘ Journal of Physiology,’ vol. 18, p. 117.