1888-89.] Mr G. N. Stewart on Electrotonic Variation. 241 
different in purpose from mine and essentially different in method, 
found that the cathodic block took time for its establishment, and 
that, when it appeared, it appeared suddenly. 
In the example given in Experiment 5 the initial positive effect 
is seen to change in 30" into a negative effect thrice as great, and 
this negative effect then gradually increases with still longer time 
of closure. 
In order to diminish, as far as possible, the irregularities in- the 
deflection, which are always a source of trouble with strong electro- 
tonic currents, especially on the anodic side, I thought of using 
the currents led off to the galvanometer from two separate nerves of 
the same frog to compensate one another, a method resembling some- 
what in principle that which Hermann has used in some of his 
polarisation work. Then, on exciting one of the nerves, one ought 
to get the stimulation effect, weakened, of course, by the extra 
resistance of the second nerve. The same battery was connected 
with both nerves, so that irregularities in the battery itself might 
be eliminated. The result was very satisfactory. 
Figs. 3 and 4 show the arrangement. 
In the arrangement of fig. 4 two nerves were placed on two 
separate sets of electrodes A, B, D ; A', B', D', a compensator (Com.) 
being introduced into the galvanometer circuit. 
Fig. 3. — G is the galvanometer ; Bat., the battery ; I, the stimulating 
electrodes. 
The pieces of nerve BD, B'D' were made as nearly as possible 
equal in length, and therefore the current would have nearly the 
same density in each. The electrotonic currents in AB, A'B' would 
be nearly equal, and they would pass through the galvanometer in 
opposite directions. The balance was completed by means of the 
compensator. 
In the arrangement of fig. 4 the polarising current passed to 
vol. xvi. 16/7/89 Q 
