116 
THE MOTION OF THE CILIA OF ANIMALCULA. May, 1890 . 
so on, until in twelve hours it will appear to have made one 
complete revolution, though in reality it has made thirteen. 
Now, imagine that our minute hand revolves at a speed of 
twelve times in a single second, and that instead of simply 
glancing at it at the right moment, which we can no longer 
do, we illuminate it with a sudden flash, leaving it in dark¬ 
ness the rest of the time, then, if we make twelve flashes a 
second, the clock-hand will appear always in the same 
position, whatever that may happen to be ; only, the flashes 
being so rapid, the eye will receive a continuous impression, 
and we shall think we see it standing still. And so, also, if 
while we give twelve flashes a second the hand revolves 
thirteen times, it will appear to us to be revolving steadily 
once round every second. 
An apparatus for producing flashes of light for this pur¬ 
pose is called a stroboscope, and was used originally for 
observing the vibrations of a stretched string. I will now 
exhibit the particular form of stroboscope which I have 
employed for studying the motion of the cilia of the animal- 
cula. I make no apology for the roughness of the instru¬ 
ment ; it is just as I made it in 1873. It consists of a board 
in which is a stout piece of wire bent over at right angles, at 
a height of about twelve inches, and pierced with a hole, 
through which this upright steel spindle may work. This 
spindle, if I must confess the truth, is half of an umbrella 
spoke, and let me tell you that you can get no better steel 
for such purposes than the spoke of a good, old-fashioned 
umbrella. The lower end is filed to a point, and works in a 
dent punched in a small piece of tin-plate. About one and 
a half inches from the bottom is a small wooden disc, which 
supports a circular card some seven inches in diameter, with 
ten narrow slits, each two degrees wide. Two leaden balls 
hang from a point just underneath the upper bearing of the 
spindle by strings of exactly equal length. Just above the 
weights there is a guide, formed of two light curved wires, 
between which the strings move freely : the whole arrange¬ 
ment being very much like the governor of a steam engine, 
with this difference, that the weights being supported by 
strings, the length of the strings, and, consequently, the rate 
of the governor can be altered by sliding up or down a little 
ring of wire, which embraces the spindle, and through 
which the cords pass. 
If I now, by taking hold of the top of the spindle, cause it 
to revolve, the balls fly out at an angle and continue to 
revolve, carrying the disc with them, until they gradually hang 
vertically once more, when the thing stops quite suddenly. 
