1876.] 



on the Leaf of Dioiisea miiscipula. 



413 



however, the marginal hairs bend inwards, and if the leaf is carefully 

 watched it may be observed that each touch is followed by a slight approach 

 of the lobes to eacli other. If the observation is continued, it is seen 

 that each approach exceeds its predecessor in extent, until at last the 

 lobes suddenly come together in the manner which is, by this time, 

 familiar to every one. 



It being our primary purpose to determine the time which elapses 

 between each several touch and the immediately resulting jerk-like 

 approach, and to measure the extent of such approach, so as to learn how 

 much it contributes to the final result, we made it our first business to 

 devise some method of measurement by which we could verify the con- 

 clusions to which we had come from rough observation, namely, that each 

 several excitation of the leaf is attended by some mechanical effect. 

 With this view, we constructed an instrument of the form shown in 

 figs. 1 & la. It consists of a hinged screw-clamp by which the leaf is 

 held, as between the thumb and fore finger. On the lower jaw of the clamp, 

 the one corresponding to the thumb, which is made of glass, the midrib 

 of the leaf rests by its under surface. The upper jaw consists of an arch 

 of copper wire, of which the curvature corresponds to that of the margins 

 of a lobe of the leaf. The two jaws meet each obher in such a way that 

 when they are brought together by the screw the two ends of the arch 

 are in apposition with the ends of the midrib close to its upper surface. 

 Each end is pierced by a pin : the points of these pins are directed 

 towards each other, so that together they serve as an axis of rotation for 

 a second similar arch of which the curvature is made a little larger 

 than the other, so that it may comprise it. From the middle of the second 

 arch a wire springs at right angles, to which a light glass lever is 

 attached. By means of this lever it can be rotated outwards on its axis, 

 and thus made to diverge from its fellow at any desired angle. 



damp ordmarily used for holding a leaf during prolonged electrical exploration. 



The instrument is used as follows : — A leaf having been placed in 

 position, that is with its midrib resting on the glass support, the two 

 arches are brought down by the screw until they all but touch the trough 

 of the leaf at opposite ends. This done, the arches are made to diverge 



2q2 



