at the edge of the dark space.

Hitherto I have spoken only of the phosphorescence of substances
placed under the negative pole. But from numerous experiments I find
that bodies will phosphoresce in actual contact with the negative
pole.

[Illustration: FIG. 24--PRESSURE = 0.25 MM. = 330 M.]

This is only a temporary phenomenon, and ceases entirely when the
exhaustion is pushed to a very high point. The experiment is one
scarcely possible to exhibit to an audience, so I must content myself
with describing it. A U-tube, shown in Fig. 25, has a flat aluminum
pole, in the form of a disk, at each end, both coated with a paint of
phosphorescent yttria. As the rarefaction approaches about 0.5
millimeter the surface of the negative pole, A, becomes faintly
phosphorescent. On continuing the exhaustion this luminosity rapidly
diminishes, not only in intensity but in extent, contracting more and
more from the edge of the disk, until ultimately it is visible only as
a bright spot in the center. This fact does not prop a recent theory,
that as the exhaustion gets higher the discharge leaves the center of
the pole and takes place only between the edge and the walls of the
tube.

[Illustration: FIG. 25.]

If the exhaustion is further pushed, then, at the point where the
surface of the negative pole ceases to be luminous, the material on
the positive pole, B, commences to phosphoresce, increasing in
intensity until the tube refuses to conduct, its greatest brilliancy