value of the intensity of stimulus as obtained from the scale-reading of a particular coil gives us no idea of the absolute intensity. It appeared desirable, nevertheless, in making quantitative experiments, to adopt some unit of stimulus in terms of which other intensities might be expressed. It would be well, moreover, to select this unit in some way not quite arbitrary, so that it might carry a significance more or less universal. The unit intensity of exciting shock which I have adopted for these reasons is that which barely induces in ourselves a perceptible sensation. The observer dips two fingers, one of each hand, into two troughs of saline solution, which are in series with the experimental Mimosa and the secondary coil. The plant tissue is interposed so as to ensure an identical current to pass through the experimental individual and the plant. The resistance offered by the plant tissue is very great; in the case of Mimosa under the usual mode of connection, it is about half a million ohms. At the beginning the secondary is placed at a great distance from the primary. The vibrating interrupter of the primary is next started and the secondary gradually pushed in, till at a certain scale-reading the observer, who is kept in ignorance of the position of the secondary, just begins to perceive the shock. This process is repeated several times in the case of the individual observer, and the mean of various consecutive readings, which ought not to differ from each other to any extent, is taken as the unit for that particular individual. The same observation is repeated with some ten different individuals, and the mean of these ten readings is finally adopted as that reading of the unit intensity which is to serve as the standard.
Though this reading cannot be regarded as absolute and invariable, yet, in the particular circumstances of the case, it is fairly definite and on the whole satisfactory. It gives us a general idea, moreover, of that intensity which will be effective in stimulating the plant, in terms of the minimal stimulus capable of evoking sensation in man. Having thus obtained the scale-reading corresponding to this unit,
