INTRODUCTION. 
13 
periphery of the segmented ring at the required rate of 
speed, and the apparatus was set in operation. 
If the series was to illustrate progressive motion, the 
model or animal would start on its journey at a more or 
less distant point from the cameras. On its approach to 
the figure 1 on the track, and as near thereto as its speed 
and the personal equation of the director allowed, an 
independent current was switched on through a magnet 
below the rod of the contact-brush. 
The action of the armature released the lower end of 
the rod on the loose collar, which, by means of a coiled 
spring, was immediately thrown into gearing with the 
already revolving shaft ; the contact-brush swept around 
the segmented ring, and made the consecutive series of 
exposures at the pre-arranged intervals of time. 
Providing it was so intended, three exposures were 
made simultaneously, one through each of the lenses 
marked 1 at each of the operating points marked respec¬ 
tively L, R, and F. At the pre-determined interval of 
time, another three synchronous exposures were made 
through each of the lenses marked 2, and so on until 
the entire series was completed. If the time had been 
accurately calculated, the successive phases were photo¬ 
graphed in precise accordance with the arrival of the 
model at the numbered places on the track, and exactly 
opposite the correspondingly numbered camera. This 
perfect uniformity of time, speed, and distance was not 
always obtained, and allowance was therefore usually 
made for a slight overlapping of the phases required to 
illustrate a complete stride or movement. 
The time-intervals of exposures varied from the 
one-hundredth of a second to several seconds. A record 
of these time-intervals was kept by the chronograph 
—a well-known instrument, used in every physiological 
laboratory—it comprises a revolving cylinder of smoke- 
blackened paper, on which, by means of successive electric 
contacts, a style is made to record the vibrations of a 
tuning-fork, while a second style marks the commence¬ 
ment of each successive exposure. The number of 
vibrations occurring between any two exposures marks 
the time. 
The tuning-fork made one hundred vibrations in a 
second of time. To ensure greater minuteness and 
accuracy in the record, the vibrations were divided into 
tenths, and the intervals calculated in thousandths of a 
second. 
For the purpose of determining the synchronous action 
of the electro-exposers while making a double series of 
exposures, the accuracy of the time-intervals as recorded 
by the chronograph, and the duration of the shortest 
exposures used in the investigation, the two cameras of 
twelve lenses each were placed side by side, and the 
exposers were connected through their respective magnets 
with the motor-clock by separate lengths of one hundred 
feet of cable. The lenses of the two cameras were 
pointed to a rapidly revolving disc of five feet diameter. 
The surface of the disc was black, with thin white threads 
radiating from the centre to the edge. 
A microscopic examination of the two series of result¬ 
ing negatives failed to prove any variation from the 
