Properties of the leaf of hiokhia. 3i 
impossible to get the after effect on the screen, although the difference was not 
larger than in the other two. On interpolating as before a length of wire correspond¬ 
ing to 0'009 D., and closing the circuit 2" after excitation, the successive readings at 
5" intervals were: +200, + off scale, +230, +60, -50, -130, -210, -260, -298, 
— 320, —345, —350, —360, &c., the reading due to over compensation being —370 ; 
.so that here, as in the other instances, the maximum difference was between 0‘01 D. 
and 0’02 D. 
Influence of repeated excitations on the intensity and duration of the after effect .— 
Whenever a freshly prepared leaf is observed with the capillary electrometer during 
the course of a succession of excitations, it is found that in the first two or three the 
negative movement of the mercurial column is followed by a positive one which equals 
or may even exceed itself in extent. As the excitations are repeated, the positive 
phase becomes smaller and smaller, until after eight or ten, it may very readily escape 
observation. 
For the purpose of obtaining more exact information on this subject, the galvano¬ 
meter and rheotome were used; for it was obvious that if in a succession of experi¬ 
ments the electrical condition of the led off surfaces could be compared, during a 
period after excitation corresponding to the maximum of the after effect, that is, the 
period during which the internal surface of the leaf is most negative, the successive 
de Sections recorded would give a good indication of any such diminution as the 
observations with the electrometer indicated. 
The period of closure chosen was between 3" and 4‘5" after excitation. The deflec¬ 
tions were, of course, all positive, but varied in extent from 175 to 21. In 21 succes¬ 
sive excitations the mean of the first seven deflections was 157, of the second seven, 
86, and of the third, 5 5‘5. In this experiment the leaf was excited at intervals of not 
less than two minutes. If the excitations are repeated as often as every half minute 
it is found that the after effect completely disappears. This can be easily shown by 
the electrometer, but can be more exactly determined by the galvanometer. The 
best way to do this is, first to make with the galvanometer an observation of the kind 
described in the last paragraph (p. 30), then to subject the leaf to a succession of excita¬ 
tions at short intervals, and to follow these by a second galvanometer observation for 
comparison. The following results are obtained:—- 
(1.) The fourth excitation of a leaf showed an after effect of which the course was 
as follows, as indicated by the negative deflections observed at 5" intervals: +300, 
+ 262, +155, +100, +74, +66, +59, &c. 
(2.) After ten effectual excitations at half minute intervals the after effect was: 
+ 160, +154, +70, +49, +40, +30, +12, &c. 
(3.) After ten more effectual excitations it was: +25, +5, 0, 0, &c. The leaf 
was then left to itself for 15 minutes and the after effect again observed. It was : 
+ 85, +130, +62, +52, + 49, +40, +36, &c.; and after ten minutes’ further rest: 
+ 100, +200, +104, +70, +62, +52, +47, &c. 
