tible movement it is seen standing
absolutely still with all marks upon it quite visible to the eye. When
Sight perception is increased up to the rate at which time may be said
to flow for any particular object we apparently get into the reality,
the permanent _now_ where motion ceases to exist as a sensation. A
tuning-fork, kept vibrating, by means of an electro-magnet, at 2000
times per second, may to our sense of sight be gradually slowed down
and, optically, brought absolutely to a standstill, for as long as
desired, and the smallest irregularity of its surface may be minutely
examined, though it continues to be heard and felt vibrating at that
enormous rate. I have made several experiments in this direction, and
some very curious facts connected with the sensation of Motion are
brought to light by means of this increase in perceptive power. If the
sense of sight is increased to 125 units per second, motion at the
rate of one inch per second is barely visible; taking the common
house-fly, whose wings vibrate about 400 times per second, its units
of perception would appear to be about two-thirds of those beats, as I
found it had no cognisance of Motion below two inches per second; you
can put your finger on any fly provided you do not approach it faster
than the above rate, it turns its head up to look at your finger but
can see no motion in it; if you approach at over three inches per
second it will always fly away before you are within a foot. I found
that a dragon-fly, whose wings vibrate about 200 times per second, had
only half the number of unit perceptions of the fly and could
apparently see motion at about one inch per second but not under. In
the converse of the above we have then the principle of a Microscope
for Time, somewhat similar to the Microscope for Space of our
laboratories. If our perception were increased sufficiently we could
slow down any motion for examination, however rapid; there would be no
difficulty in following a lightning flash or even arresting its
visible motion for purposes of investigation without interfering with
the natural sequence of cause and effect.

If, on the other hand, our perception were decreased below six times
per second, all motion would be accelerated, until with perception
reduced to one unit in twenty-four hours the sun would appear only as
a band across the sky, and we could not follow its motion any more
than, as we have seen, we could follow the point of a red-h