STANDARDISATION OF STIMULUS 49 



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 solu- 

 tion, 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 consecu- 

 tive 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, 



