PROPERTIES OP THE LEAP OP DIOMA. 
33 
giving an account of them it will be useful to consider what we should expect. This 
depends mainly on the question whether, in the transmission of the excitatory effect 
from the seat of excitation to the opposite lobe, time is lost or not. In the funda¬ 
mental experiment we have evidence that the excitatory effect travels. For when we 
excite the left lobe and the right responds, some time is without doubt occupied in 
transmission. If, as Professor Munk believes, the transmission is so extremely rapid 
that the excitatory change may he said to begin at the same moment at all parts, the 
case would resolve itself into the hypothetical one from which we started, viz.: that 
of excitation taking its rise at the same moment from both leading off contacts; and 
this being so, any galvanometric effect observed must be due to one of the two causes 
referred to above, namely, either to unequal intensity or to unequal duration of the 
electrical response of the two sides of the leaf. If, in addition to these inequalities, 
we have to consider the influence of propagation (i.e., if the time lost in propagation 
is sufficiently considerable to tell), the interpretation of the galvanometric effect 
observed in an experiment in the form represented in fig. 13 becomes extremely 
complicated; for the modifications severally due to loss of time and to difference of 
intensity would interfere with each other in a way very difficult to estimate. It will 
be shown in a subsequent section that the rate of propagation in the excitatory dis¬ 
turbance is great, as compared with the duration of the excitatory effect; in other 
words, that its wave-length is considerable. This being so, the question whether or 
not the modification due to loss of time will show itself, will depend on the abruptness 
with which the disturbance commences, as will be seen from the following diagrams. 
If, for example, we suppose that the excitatory electromotive changes are so directed 
that the led off surface on either side becomes negative to the midrib, and that the 
rise of negativity on the right side is represented in two different leaves by the curve 
a, that of the left side by the curve b, the galvanometric effect during the period repre¬ 
sented -will be expressed in each figure by the shaded area, the depth of which is greater 
in the first case than in the second; so that if (as is represented in fig. 12, C) it were 
further diminished by a greater abruptness in the rise of negativity on the left side, 
it might escape observation. And it is easy to see that if, in addition to beginning 
more abruptly the excitatory disturbance on the left side were more intense, the 
influence of the loss of time in propagation would lose itself entirely in that of 
inequality. The two last suppositions (fig. 12, B and C) appear to afford the key to 
the understanding of what is actually observed when a leaf is led off by symmetrical 
points and excited near one of them. In this case there is always an excitatory effect, 
proving that the two lobes do not respond equally. Further, we have found that so 
far from its being true, as asserted by Professor Munk, that it makes no difference 
what spot is chosen as the seat of excitation, it never happens that variations of the 
same character are obtained when, in two successive experiments, or in two series of 
experiments, the right and left lobe are alternately excited. At first sight it seemed 
as if this difference were referable to propagation, but more careful consideration has 
MDCCCLXXXII. F 
