PREFACE. 
3 
and made stereoscopic pictures. Selecting from these 
stereographs a suitable number of phases to reconstitute 
a full stride, he placed the appropriate halves of each, 
respectively, in one of the scientific toys called the 
zoetrope, or the wheel of life—an instrument originated 
by the Belgian physicist Plateau, to demonstrate the 
persistency of vision. These two zoetropes were geared, 
and caused to revolve at the same rate of speed; the 
respective halves of the stereographs were made simul¬ 
taneously visible, by means of mirrors—arranged on the 
principle of Wheatstone’s reflecting stereoscope—succes¬ 
sively and intermittently, through the perforations in the 
cylinders of the instruments, with the result of a very 
satisfactory reproduction of an apparently solid miniature 
horse trotting, and of another galloping. 
Pursuing this scheme, the author arranged, in the 
same consecutive order, on some glass discs, a number 
of equidistant phases of certain movements; each series, 
as before, illustrated one or more complete and recurring 
acts of motion, or a combination of them : for example, 
an athlete turning a somersault on horseback, while the 
animal was cantering; a horse making a few strides of 
the gallop, a leap over a hurdle, another few strides, 
another leap, and so on; or a group of galloping 
horses. 
Suitable gearing of an apparatus constructed for the 
purpose caused one of these glass discs, when attached 
to a central shaft, to revolve in front of the condensing 
lens of a projecting lantern, parallel with, and close to 
another disc fixed to a tubular shaft which encircled 
the other, and around which it rotated in the contrary 
direction. The latter disc was of sheet-metal, in which, 
near its periphery, radiating from its centre, were long 
narrow perforations, the number of which had a definite 
relation to the number of phases in the one or more lines 
of motion on the glass disc—the same number, one or 
two more, or one or two less—-according to the sequence 
of phases, the intended direction of the movement, or the 
variations desired in the apparent rate of speed. 
The discs being of large size, small portions only of 
their surfaces—showing one phase of each of the circles 
of moving animals—were in front of the condenser at the 
same instant. 
To correct the apparent vertical extension of the 
animals when seen through the narrow openings of the 
metal disc on its revolution in such close proximity to, and 
in the reverse direction of the glass disc, the photographs 
on the latter, after numerous experiments, were ultimately 
prepared as follows :—- 
A flexible positive was conically bent inwards, and 
inclined at the necessary angle from the lens of the 
copying camera to ensure the required horizontal elongation 
of the animal while the straight line of ground corre¬ 
sponded with the curvature of the intended ground-line 
of the glass disc, towards the periphery of which the 
feet of the animals were always pointed. 
A negative was then made of this phase, and negatives 
of the other phases, in the same manner. All the negatives 
required for that particular subject were then consecutively 
arranged, equidistantly, in a circle, on a large sheet of 
