LATENT PERIOD AND REFRACTORY PERIOD 265 



hundredths of a second. The experimental method that I 

 am about to describe would, however, enable us, if necessary, 

 to make determination of time-intervals of one-tenth this 

 magnitude, the question being only one of using a recording 

 drum with the requisite increased speed. The drum used for 

 these experiments was one constructed by Verdin, and 

 provided with a very perfect governor. Some little time 

 elapses after starting the drum before it acquires uniform 

 speed, which it afterwards maintains, however — at least 

 during the short time required for the experiment — with 

 great perfection. The record is not taken until this uniform 

 condition is attained. The mirror of the Optic l.ever throws 

 a spot of light upon a sensitive photographic film, wrapped 

 round the revolving drum. In order to produce records of 

 the required rapidity, I employ sunlight, proceeding from a 

 pinhole, which after reflection from the mirror of the Optic 

 Lever falls on the drum, appropriately focussed by means of 

 a condensing lens placed at the end of a focussing tube, as 

 seen in the figure. It is understood that, the record to be 

 obtained being photographic, this experiment is carried out 

 in a dark room, the sunlight required for the record being 

 directed upon the pinhole by a heliostat outside. 



The stimulus consists of a single strong break-shock, 

 from a Ruhmkorff's coil, one electrode of which, by means of 

 non-polarisable connections, is attached to the pulvinus of a 

 leaf, and the other to the main stem lower down, of a speci- 

 men of Mimosa. The shock is applied by the recording 

 drum itself, at a particular moment in the course of its 

 revolution ; and at the same instant the curve of response 

 begins to record itself automatically. These two acts — of 

 imparting stimulus, and of opening a shutter by which the 

 recording ray of light is allowed to fall upon the moving film 

 — are performed simultaneously ; and both alike are initiated 

 by the stroke of a rod which is fixed to the axis of the drum 

 underneath. 



This rod, which I shall designate as the striker, at a certain 

 period in the revolution of the drum, impinges upon a balanced 



