he rim of a wheel rotating about
a horizontal axis with great velocity. If the luminosity travelled with
infinite speed from one electrode to the other, the image on the film
would be a horizontal line. If, however, the speed with which the
luminosity travelled between the electrodes was comparable with the
speed of the film, the line would be inclined to the horizontal, and by
measuring the inclinations we could find the speed at which the
luminosity travelled. In this way Schuster and Hemsalech showed that
when an oscillating discharge passed between metallic terminals in air,
the first spark passes through the air alone, no lines of the metal
appearing in its spectrum. This first spark vaporizes some of the metal
and the subsequent sparks passing mainly through the metallic vapour;
the appearance of the lines in the film shows that the velocity of the
luminous part of the vapour was finite. The velocity of the vapour of
metals of low atomic weight was in general greater than that of the
vapour of heavier metals. Thus the velocity of aluminium vapour was 1890
metres per second, that of zinc and cadmium only about 545. Perhaps the
most interesting point in the investigation was the discovery that the
velocities corresponding to different lines in the spectrum of the same
metal were in some cases different. Thus with bismuth some of the lines
indicated a velocity of 1420 metres per second, others a velocity of
only 550, while one ([lambda] = 3793) showed a still smaller velocity.
These results are in accordance with a view suggested by other phenomena
that many of the lines in a spectrum produced by an electrical discharge
originate from systems formed during the discharge and not from the
normal atom or molecule. Schuster and Hemsalech found that by inserting
a coil with large self induction in the primary circuit they could
obliterate the air lines in the discharge.

Schenck, by observing the appearance presented when an alternating
current, produced by discharging Leyden jars, was examined in a rapidly
rotating mirror, found it showed the following stages: (1) a thin bright
line, followed in some cases at intervals of half the period of the
discharge by fainter lines; (2) bright curved streamers starting from
the negative terminal, and diminishing rapidly in speed as they receded
from the cathode; (3) a diffused glow lasting for a much longer period
than either of the preceding. These constituents gave out quite
diff