near A' is the only stable state. Thus if the BC part of the curve
corresponded to a luminous discharge and the A part to a dark
discharge, we see that if the electromotive force is kept constant
there is a minimum value of the current for the luminous discharge. If
the current is reduced below this value, the discharge ceases to be
luminous, and there is an abrupt diminution in the current.

_Cathode Rays._--When the gas in the discharge tube is at a very low
pressure some remarkable phenomena occur in the neighbourhood of the
cathode. These seem to have been first observed by Plucker (_Pogg. Ann._
107, p. 77; 116, p. 45) who noticed on the walls of the glass tube near
the cathode a greenish phosphorescence, which he regarded as due to rays
proceeding from the cathode, striking against the sides of the tube, and
then travelling back to the cathode. He found that the action of a
magnet on these rays was not the same as the action on the part of the
discharge near the positive electrode. Hittorf (_Pogg. Ann._ 136, p. 8)
showed that the agent producing the phosphorescence was intercepted by a
solid, whether conductor or insulator, placed between the cathode and
the sides of the tube. He regarded the phosphorescence as caused by a
motion starting from the cathode and travelling in straight lines
through the gas. Goldstein (_Monat. der Berl. Akad._, 1876, p. 24)
confirmed this discovery of Hittorf's, and further showed that a
distinct, though not very sharp, shadow is cast by a small object placed
near a large plane cathode. This is a proof that the rays producing the
phosphorescence must be emitted almost normally from the cathode, and
not, like the rays of light from a luminous surface, in all directions,
for such rays would not produce a perceptible shadow if a small body
were placed near the plane. Goldstein regarded the phosphorescence as
due to waves in the ether, for whose propagation the gas was not
necessary. Crookes (_Phil. Trans._, 1879, pt. i. p. 135; pt. ii. pp.
587, 661), who made many remarkable researches in this subject, took a
different view. He regarded the rays as streams of negatively
electrified particles projected normally from the cathode with great
velocity, and, when the pressure is sufficiently low, reaching the sides
of the tube, and by their impact producing phosphorescence and heat. The
rays on this view are deflected by a magnet, because a magnet exerts a
force on a charged moving bod