also pulsating at a definite enormous rate; we can at will
increase their motion by heat or reduce by cold; if our touch
perception were sensitive enough we should feel those motions and
should not have the sensation of a solid. We have a similar case of
limitation in our other senses, which we shall grasp better in another
View through our Window. We can hear beats only up to fifteen in a
second, beyond that number they give the sensation of a musical or
continuous sound. In our sense of sight we can see pulsations or
intermittent flashes up to only six in a second, beyond that number
they give the sensation of a continuous light; a gas jet, if
extinguished and relit six times in a second, can be seen to flicker,
but beyond that rate is to our sense of sight a steady flame. The
effect may also be shown by making the top of a match red-hot; when
stationary or moving slowly, it is a point of light, but, moved
quickly, it becomes a continuous line of light.

Even apart from our senses we find Motion giving the characteristics
of solidity: a wheel with only a few spokes, if rotated quickly
enough, becomes quite impermeable to any substance, however small,
thrown at it; a thin jet of water only half an inch in diameter, if
discharged at great pressure equivalent to a column of water of 500
metres, cannot be cut even with an axe, it resists as though it were
made of the hardest steel; a thin cord, hanging from a vertical axis,
and being revolved very quickly, becomes rigid, and if struck with a
hammer it resists and resounds like a rod of wood; a thin chain and
even a loop of string, if revolved at great speed over a vertical
pulley, becomes rigid and, if allowed to escape from the pulley, will
run along the ground as a hoop.

Now with regard to this limit of time perception, which gives us the
phenomenon of Solidity, I have lately been able to devise an
arrangement which, acting as a microscope for Time, gives the
sensation of an increase in sight perception up to several thousand
units per second; it is based on the fact that though the eye can only
see six times per second it can see for the one-millionth part of a
second. An example of this is the well-known experiment of seeing a
bullet in its flight; the bullet makes electrical connection resulting
in a spark which illuminates the bullet when opposite the eye. The
electrical spark exists only for the millionth of a second, and as the
bullet in that time has no percep