PROPERTIES OE THE LEAP OE DIORhEA. 
29 
In 1876 we used an arrangement by which (1) the act of excitation (if mechanical, 
the depressing of a lever which brought a camel-hair pencil into contact with one of 
the sensitive hairs; if electrical, the breaking of the primary circuit of the induction 
apparatus) recorded itself on a blackened cylinder revolving at a rapid rate by clock-¬ 
work ; and (2) the record of the response was inscribed on the same cylinder. 
In the first record, the inscription of which was effected with the aid of Deprez’ 
electro-magnetic “ signal,” or chronograph, the loss of time was inappreciable. The 
response was the motion of the mercurial column of the capillary electrometer. This 
was observed by one of us (Mr. Page), who recorded the event on the cylinder 
by closing the current through the “signal” which the other had broken in the act of 
excitation. The interval between break and make of the signal current comprised : 
(1) the physiological delay to be measured; (2) the instrumental delay (that is the 
time lost, if any, in moving the mercurial column); and (3) the “ personal time” 
or “ reaction time” of the observer. The last we had no difficulty in eliminating, for 
Mr. Page’s personal time was subject to very slight variation. The second we 
had not taken into account at all, assuming it to be non-existent. The consideration 
that excitatory difference in Dionaea probably begins gradually, and that if so 
the motion of the column is slow, and may therefore not be perceived until a longer 
period than the normal time after it has actually begun, led us later to conclude that 
our method was inadequate for the purpose, and that the only way by which the 
question could be settled was by the use of the rheotome and galvanometer. 
The after effect .—The change which has been above described as occupying the 
greater part of the first 2" after excitation is followed by a period during which the 
surfaces are nearly, or absolutely, in the same condition electrically as they were before 
excitation, but sooner or later a secondary effect begins, which I propose to call the 
after effect. During the after effect the internal surface becomes negative to the 
external. 
In order to bring the after effect into view, the rate of the rheotome must be 
reduced and the periods of closure much lengthened. In our first observation the rate 
of revolution was 1 in 78", and the closing time 1*4". The leaf was excited as before 
at intervals of three minutes, without being placed in the chamber, the temperature of 
the room being 18° C., the mode of excitation being the same as in the previous series. 
After the first excitation the galvanometer circuit was closed between 1*3" after excita¬ 
tion, and 2 - 6". In this case the deflection was negative, for, in consequence of the rela¬ 
tively low temperature at which the observation was taken, the first phase of the 
measurement was made in the moist chamber at 32° 0.—a result which exceeds the truth by about 0-05". 
Assuming that the error is to be set down, not of course to loss of time in transmission of the effect to 
the mercurial column, but to the lengthening of the “ reaction time ” when the motion to be perceived 
is gradual, it renders all the observations made by signalling with the electrometer useless for the 
purpose of measuring the duration of the delay, but does not interfere with the use of this method for 
other purposes, particularly for determining which of the two similar electrical responses occurs earliest, 
