102 MR. G. J. BURCH OR THE TTME-RELATIONS OF THE 
analysis is appended, the normal excursion photographed for the purpose being 
No. 289 (fig. 5), and the value of the constant multiplier was 8’5. 
The next step is to place the glass negative of the muscle-curve upon the measuring 
instrument (fig. 3), carefully centreing it so as to ensure its occupying a position 
exactly corresponding to that in which it was placed on the pendulum of the photo¬ 
graphic apparatus. If this is not done, the results will be falsified. The rate of 
movement has then to be determined. When, as in the cases given here, the angle 
subtended by the entire excursion is small, no appreciable error is introduced by 
employing a tangent scale instead of measuring the arc. This being so, a very simple 
method is available, whereby the hundredths of a second, given by the time-record on 
the photograph, can be subdivided into thousandths of a second. The index of the 
carrier (fig. 3), is a straight edge fixed radially, and the tangent scale, which is 
divided into millimetres, is placed at such a distance from the centre 0, that ten of 
its divisions shall be passed over by the index when the carrier is shifted through a 
space corresponding to one-hundredth of a second, as indicated by the coincidence of 
two successive apices of the time-record on the photograph with the radial line DO. 
The scale having been fixed in this position, the actual measurements are proceeded 
with. The instant of excitation, marked by the upward movement of the signal arm, 
is taken as the zero of time. This portion of the photograph is brought over the line 
DO, and the reading of the tangent scale is noted. The distance of the electrometer 
record at that time from the circular edge of the photograph is measured ; this gives 
the position of zero potential. The next thing is to find the time at which the menis¬ 
cus began to move. For this purpose, the rod E is laid along the line DO, and the 
line aa, on the glass plate F, placed accurately as a tangent to the curve. The 
carrier is then shifted until a point is reached when the curve commences to rise, and 
the time at which this takes place is noted on the scale. 
Next, the carrier is further shifted in the same direction through a space corre 
sponding to O’OOOS sec. 
There are now two operations to be performed : — 
(a.) To measure the subnormal of the curve at this point. The method of doing 
this has been already described. The result may be written 
Subnormal = cy. 
(b.) To measure how far the meniscus has moved from its original zero position 
for t — 0. 
This is done by taking the distance of the curve at this point from the edge of the 
photograph, which is used as a reference circle, and subtracting the result from the 
zero distance already ascertained. The difference may be written 
Movement = l. 
It is most convenient to express the total indicated difference of potential in terms 
