rt conical bright part or root appeared at the middle
part of the ball projecting directly from it, which, at a little distance
from the ball, broke out suddenly into a wide brush of pale ramifications
having a quivering motion, and being accompanied at the same time with a
low dull chattering sound.

1427. At first the brush seems continuous, but Professor Wheatstone has
shown that the whole phenomenon consists of successive intermitting
discharges[A]. If the eye be passed rapidly, not by a motion of the head,
but of the eyeball itself, across the direction of the brush, by first
looking steadfastly about 10 deg. or 15 deg. above, and then instantly as much
below it, the general brush will be resolved into a number of individual
brushes, standing in a row upon the line which the eye passed over; each
elementary brush being the result of a single discharge, and the space
between them representing both the time during which the eye was passing
over that space, and that which elapsed between one discharge and another.

[A] Philosophical Transactions, 1834, p. 586.

1428. The single brushes could easily be separated to eight or ten times
their own width, but were not at the same time extended, i.e. they did not
become more indefinite in shape, but, on the contrary, less so, each being
more distinct in form, ramification, and character, because of its
separation from the others, in its effects upon the eye. Each, therefore,
was instantaneous in its existence (1436.). Each had the conical root
complete (1426.).

1429. On using a smaller ball, the general brush was smaller, and the
sound, though weaker, more continuous. On resolving the brush into its
elementary parts, as before, these were found to occur at much shorter
intervals of time than in the former case, but still the discharge was
intermitting.

1430. Employing a wire with a round end, the brush was still smaller, but,
as before, separable into successive discharges. The sound, though feebler,
was higher in pitch, being a distinct musical note.

1431. The sound is, in fact, due to the recurrence of the noise of each
separate discharge, and these, happening at intervals nearly equal under
ordinary circumstances, cause a definite note to be heard, which, rising in
pitch with the increased rapidity and regularity of the intermitting
discharges, gives a ready and accurate measure of the intervals, and so may
be used in any case when the discharge is heard, even t