equence of the rapidity of the motion some optical device is
necessary to render apparent the phenomena attending the disintegration
of a jet. Magnus employed a rotating mirror, and also a rotating disk
from which a fine slit was cut out. The readiest method of obtaining
instantaneous illumination is the electric spark, but with this Magnus
was not successful. The electric spark had, however, been used
successfully for this purpose some years before by H. Buff (_Liebigs
Ann._ lxxviii. 1851), who observed the _shadow_ of the jet on a white
screen. Preferable to an opaque screen is a piece of ground glass, which
allows the shadow to be examined from the farther side (Lord Rayleigh).
Further, the jet may be very well observed directly, if the illumination
is properly managed. For this purpose it is necessary to place it
between the source of light and the eye. The best effect is obtained
when the light of the spark is somewhat diffused by being passed (for
example) through a piece of ground glass.

The spark may be obtained from the secondary of an induction coil, whose
terminals are in connexion with the coatings of a Leyden jar. By
adjustment of the contact breaker the series of sparks may be made to
fit more or less perfectly with the formation of the drops. A still
greater improvement may be effected by using an electrically maintained
fork, which performs the double office of controlling the resolution of
the jet and of interrupting the primary current of the induction coil.
In this form the experiment is one of remarkable beauty. The jet,
illuminated only in one phase of transformation, appears almost
perfectly steady, and may be examined at leisure. In one experiment the
jet issued horizontally from an orifice of about half a centimetre in
diameter, and almost immediately assumed a rippled outline. The
gradually increasing amplitude of the disturbance, the formation of the
elongated ligament, and the subsequent transformation of the ligament
into a spherule, could be examined with ease. In consequence of the
transformation being in a more advanced stage at the forward than at the
hinder end, the ligament remains for a moment connected with the mass
behind, when it has freed itself from the mass in front, and thus the
resulting spherule acquires a backwards relative velocity, which of
necessity leads to a collision. Under ordinary circumstances the
spherule rebounds, and may be thus reflected backwards and forwards
s