that there is a flash, have
travelled just a little too far, and so they will all appear to be
moving slowly forwards. Now let us try the experiment. There is the
electric lantern sending a powerful beam of light on to the screen. This
I bring to a focus with a lens, and then let it pass through a small
hole in a piece of card. The light then spreads out and falls upon the
screen. The fountain of water is between the card and the screen, and so
a shadow is cast which is conspicuous enough. Now I place just behind
the card a little electric motor, which will make a disc of card which
has six holes near the edge spin round very fast. The holes come one
after the other opposite the hole in the fixed card, and so at every
turn six flashes of light are produced. When the card is turning about
21-1/2 times a second, then the flashes will follow one another at the
right rate. I have now started the motor, and after a moment or two I
shall have obtained the right speed, and this I know by blowing through
the holes, when a musical note will be produced, higher than the fork if
the speed is too high, and lower than the fork if the speed is too low,
and exactly the same as the fork if it is right.

To make it still more evident when the speed is exactly right, I have
placed the tuning-fork also between the light and the screen, so that
you may see it illuminated, and its shadow upon the screen. I have not
yet allowed the water to flow, but I want you to look at the fork. For a
moment I have stopped the motor, so that the light may be steady, and
you can see that the fork is in motion because its legs appear blurred
at the ends, where of course the motion is most rapid. Now the motor is
started, and almost at once the fork appears quite different. It now
looks like a piece of india-rubber, slowly opening and shutting, and now
it appears quite still, but the noise it is making shows that it is not
still by any means. The legs of the fork are vibrating, but the light
only falls upon them at regular intervals, which correspond with their
movement, and so, as I explained in the case of the water-drops, the
fork appears perfectly still. Now the speed is slightly altered, and, as
I have explained, each new flash of light, coming just too soon or just
too late, shows the fork in a position which is just before or just
behind that made visible by the previous flash. You thus see the fork
slowly going through its evolutions, though of cours