236 
DR, A. D. WALLER AND MR. E. W. RETD ON THE 
a black projection into a light area is recorded on the negative. Of course this is 
again reversed in the positi ve prints of such negatives.* 
We did not, as in the case of the galvanometer, adopt only one mode of connection 
of the electrometer, but varied it so that sometimes A was in connection with H 2 S0 4 , 
sometimes with Hg; this we considered advisable, seeing that the movements of the 
capillary column do not take place with equal facility in both directions, but more 
easily towards than from the end of the tube. But, to preserve uniformity in the 
registration of our results, we give them as if the connection were throughout as in 
the diagram ; negativity of A is then denoted by a north variation, negativity of B 
by a south variation (apparent variation in field of microscope). 
(a.) Of spontaneous contractions .—Our first trials showed at once that the 
readings of the electrometer agree with those of the galvanometer, but that, as might 
be expected, rapid changes are read by the former instrument which escape observa¬ 
tion by the latter. Thus, c.g., in Experiment 42 the spontaneous variation galvano- 
metrically was N, while electrometrically it was SN, and in Experiment 44 it was 
galvanometrically S, electrometrically NS. The superiority of the electrometer is not;-, 
however, fully made use of without having recourse to the photographic method, by 
which a permanent record of the variations is made. Whereas by the swinging 
magnet or swinging coil of a galvanometer magnitude and duration of electromotive 
changes, such as occur with action of the heart, are compounded and cannot be 
separated, their separate estimation by the electrometer is to some extent possible ; 
how far this is so we do not know, doubtless the curve described by the mercury 
column does not give the absolute course as to duration and magnitude of a very 
rapid electromotive change ; different electrometers, or the same electrometer at 
different times, may have a different amount of deforming effect. This is, however, 
* The following key will be of service for the reading of onr photographs :— 
With A to 1LSO, and B to Hg (as in diagram).— If A becomes negative to B, the mercury moves 
towards the end of the capillary, i.e., northwards in the field of the microscope; the photographic 
negative shows a light projection into a dark area, the positive shows a dark projection into a light area. 
If B becomes negative to A, all is reversed ; the mercury moves southwards in the electrometer image, and 
the negative shows a dark projection into a light area, the positive shows a light projection into a dark 
area. 
With A to Hg and B to H 3 S0 4 .— If A becomes negative to B, the photographic negative shows a dark 
projection into a light area. If B becomes negative to A, the photographic negative shows a light pro¬ 
jection into a dark area. In all our photographs the upper border of the figure corresponds with the 
north of the microscope field; the time-tracing is recorded at this border. In the woodcuts, which 
are the reproduction of photographic positive prints, the lower border of the figure corresponds with 
the north of the microscopic field; the time-tracing is recorded at this border. The black portion 
of the woodcuts reproduces blocking of light by the mercury column, but in the woodcuts North 
movement of the mercury is represented by black projection towards the time-tracing, South movement 
by white projectiou in the opposite direction. All photographs read from left to right. The rates 
of movement of the sensitive surface are recorded on each plate. Their actual values are given with 
photos. 3 and 4. 
