May, 1890. the motion of the cilia of animalcula. 117 
What I wish to point out is, that the rate of revolution is 
practically constant so long as the balls are swinging freely. 
If I pinch the spindle so as to check it, the balls come nearer 
together, but the disc, in spite of the friction, goes no slower. 
Or, if my experiment is not finished and I perceive that the 
balls are approaching the vertical, I need only give it an 
extra spin, and the balls fly out again, but it goes no faster even 
while my hand is on it, unless I continue spinning it until the 
cords press against the ends of the wire guides. The only 
way of altering the rate is by shortening the strings in the 
way I have mentioned with the slider. 
A little consideration will show the reason of this. A 
weight hanging on a string forms a simple pendulum, in 
which the time of vibration is independent of the amplitude 
of the swing, and depends only on the length of the string. 
Now a weight hanging on a string may not always swing 
to and fro in a straight line, but may describe an ellipse, 
which will, however, make no difference in the time it takes 
to come round again to the same place. Imagine, then, such 
a weight swinging not merely in an ellipse but round and 
round in a circle, and you have the principle upon which this 
apparatus is constructed. If these cords were 89 inches and 
a fraction long they would swing, as a pendulum, to and fro 
in exactly two seconds; or revolving, they would turn the 
disc once round in two seconds. Being, as they are, one 
quarter of that length, they cause it to revolve once 
every second, and as I can by the slider reduce them 
to one-quarter of their present length they will then 
maintain a practically constant rate of two revolutions per 
second for nearly three minutes without a fresh impulse. 
My disc has ten slits in its circumference. I am therefore 
able to get from ten to twenty flashes of light every second. 
I have described this instrument minutely because it costs 
practically nothing, and is sufficiently accurate to take the 
place of a very expensive piece of apparatus. 
It should be made as light as possible, so as to concen¬ 
trate all the weight in the leaden balls. The light is thrown 
by a bull’s eye condenser upon a small mirror placed above 
the disc, and is reflected downwards through one of the slits 
upon another similar mirror underneath it, whence it proceeds 
to the microscope, which must be furnished with a good Abbe 
condenser, so as to obtain a maximum intensity of illumina¬ 
tion. When this has been satisfactorily adjusted the machine 
is set spinning, and the rate altered by the slider—without 
stopping it—until the cilia of the creature under observation 
appear to stand still. The number of flashes per second 
