118 
THE MOTION 01’ THE CILIA OF ANIMALCULA. May, 1890. 
which produces this effect may either be calculated from the 
length of the strings below the slider, or more simply by 
counting the revolutions during 100 seconds, multiplying by 
the number of slits in the disc, and then omitting the last 
two figures. 
Now, as I want to make this paper as much a study of 
apparatus as of cilia, I will proceed to discuss the question 
which ought always to be asked with regard to any invention, 
namely : What are the conditions of accuracy, and how large 
are the errors of this method ? The cilia are in many cases 
fine, like bristles—shall we see them as bristles ? 
Let us return to the illustration of the clock hand. Between 
one flash and the next the hand moves once round the 
dial, but during the flash it also moves. Now my ten slits 
upon this disc are 86 degrees apart, and each one occupies 
2 degrees, so that the proportion of light to darkness is such 
that during the flash the clock hand would have turned 
through an angle equal to 8 minutes and 20 seconds clock 
measure. It would appear then not as a sharp line, but 
blurred into a fan shape. It will at once occur to you that 
to get good definition we must have short flashes, and there¬ 
fore narrow slits; but then another difficulty comes in, 
namely, that in reducing the duration of the flash we reduce 
the quantity of light that reaches the eye, and this is very 
nearly the same as reducing its brilliancy, so that a limit 
is soon reached beyond which there is not light enough to see 
by. Evidently the remedy for this is to have a brighter light, 
and, in fact, we want the brightest possible light for the 
shortest possible time. What that light is, the familiar 
metaphor both for brevity and brilliancy will at once suggest 
— a flash of lightning. Thanks to the inventions of our 
century, it is possible for us to secure a succession of such 
flashes. We need only use a metal disc with slits, and con¬ 
nect one pole of our battery with it while the other is con¬ 
nected with a wire rubbing against the disc. Then, as each 
slit passes, we shall get what is known as the break spark, 
which will be more intense if a large coil of wire is included 
in the circuit. Unfortunately I am not able to show you this 
to-night, as I cannot procure in this part of the Museum a 
current strong enough. I may, however, say that the defini¬ 
tion obtainable in this way is perfect, and the effect extremely 
beautiful. 
I now pass to the third division of my subject, namely, 
the observations made with this apparatus. I must premise 
that what I have to say does not refer to the small cilia upon 
the bodies of some animalcula, but rather to the larger and 
