MOTION IN ANIMALS AND PLANTS. 335 



excursions of the meniscus of the mercury column as a sensitive plate 

 moves rapidly past the slit on which it is projected, each upward move- 

 ment of the image indicating that the surface of contact connected 

 with the mercurj^ has become at that moment positive to the other. 



I do not propose to give this afternoon even the shortest description 

 of the instrument, and I should not occupy time in explaining why it 

 answers my purpose so perfectly were it not that with the exception 

 of Professor Einthoven and Dr. R. Du Bois-Reymond, the leading 

 authorities on the other side of the Channel, and particularly Professor 

 Hermann, have condemned it as an instrument of which the defects 

 are essential and irremediable. As I have answered these criticisms 

 elsewhere, I need only say here that for the investigation of the order 

 and duration of a rapid succession of electrical changes such as these 

 with which we are now. concerned the instrument surpasses all others 

 and that by means of it my colleague, Professor Gotch, has, with Mr. 

 Burch's aid, successfully photographed phenomena in nerve of which 

 the very existence could not be demonstrated a few years ago.^ 



The purposes to which we apply it are (1) for the measurement of 

 intervals of time between electrical changes which succeed each other 

 with great rapidity, and (2) the obtaining an estimate of their relative 

 intensities. The properties which make it so invaluable to us are (1) 

 that it responds to the action of a current promptly, beginning when 

 the current is closed, and indicating every change in its strength or 

 direction without measurable loss of time; (2) that met. j!?f/r., the rate 

 of ascent is proportional to the electromotive force of the current 

 which produces it, and (3) that the instrument can be graduated* and 

 its graduation verified by comparison with instruments of greater pre- 

 cision, and thus used for the measurement of differences of potential 

 of longer duration. 



The diagrams 1, 2, and -i illustrate the bearing of these three prop- 

 erties on the cases we have to investigate. As we shall see, a muscle 

 can be brought into action either by an instantaneous stimulus, by a 

 series of stimuli, or by continuous stimulation. Each of these has its 

 mechanical and its electrical response. 1 will anticipate so far as to say 

 that the three forms of electrical response correspond to the three 

 forms of mechanical. They correspond to the changes indicated by 

 the black lines in the three diagrams. I will further premise that all 



^Full information relating to the instrument will be found in Mr. Burch's work. 

 The Capillary Electrometer in Theory and Practice, and his papers in the Proceed- 

 ings (vol. 48, 1890) and Transactions (A, vol. 183, 1892) of the Royal Society. A very 

 perfect method of recording the excursions of the electrometer photographically and 

 of interpreting the curves was described by Professor Einthoven in Pfl tiger's Archiv 

 in 1894, and applied by him to the investigation of the electromotive phenomena of 

 the human heart. It need scarcely be added that the two methods are the same- in 

 principle. An important paper has also recently been published by Dr. R. du Bois- 

 Reymond in the Archiv f. An. u. Physiol., 1898, p. 516. 



