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ACTION OJ¥ PHOTOGRAPHIC PLATES 5

ordinary commercial plates they would be the most suitable
for the study of the positive rays; all the plates of this kind
I have tried, however, have been too streaky to permit the
determination of faint lines with. any certainty. The plates
known as " Imperial Sovereign " give very good results.

The positive rays gradually remove a thin deposit of metal
from the part of walls of the tube against which they
strike, Such thin deposits can readily be produced by run-
ning an electric discharge through the tube when it contains
gas at a low pressure, using for the cathode a piece of the
metal it is wished to deposit on the glass. The metal cathode
" splutters ^JJ and the metal is deposited as a thin layer on the
glass near the cathode.

DOUBLE CATHODES.

Goldstein^l found that positive rays came freely from the
space between two parallel plates metallically connected to-
gether and used as a cathode for the discharge through gas at a
low pressure. Cathode rays also come from this region, and
the discharge from a cathode of this kind, through a gas where
there is a marked difference in colour between the luminosity
produced by the cathode and positive rays, presents some very
interesting features. Hydrogen, and to a still greater degree
helium and neon, are suitable gases for this purpose. When a
cathode formed of two parallel equilateral triangles connected to-
gether by a wire is used for the discharge through helium at a
low pressure, the discharge near the cathode has the appearance
represented in Fig. 3. From the points of the triangle stream
pencils of luminosity showing the characteristic red colour of
the positive rays in helium, while the middle points of the sides

•* Goldstein, " Phil. Mag." VI, p. 372,1908.



	ACTION OJ¥ PHOTOGRAPHIC* PLATES* 5 ordinary commercial plates they would be the most suitable for the study of the positive rays; all the plates of this kind I have tried, however, have been too streaky to permit the determination of faint lines with. any certainty. The plates known as " Imperial Sovereign " give very good results. The positive rays gradually remove a thin deposit of metal from the part of walls of the tube against which they strike, Such thin deposits can readily be produced by run- ning an electric discharge through the tube when it contains gas at a low pressure, using for the cathode a piece of the metal it is wished to deposit on the glass. The metal cathode " splutters ^JJ and the metal is deposited as a thin layer on the glass near the cathode.

	DOUBLE CATHODES. Goldstein^l found that positive rays came freely from the space between two parallel plates metallically connected to- gether and used as a cathode for the discharge through gas at a low pressure. Cathode rays also come from this region, and the discharge from a cathode of this kind, through a gas where there is a marked difference in colour between the luminosity produced by the cathode and positive rays, presents some very interesting features. Hydrogen, and to a still greater degree helium and neon, are suitable gases for this purpose. When a cathode formed of two parallel equilateral triangles connected to- gether by a wire is used for the discharge through helium at a low pressure, the discharge near the cathode has the appearance represented in Fig. 3. From the points of the triangle stream pencils of luminosity showing the characteristic red colour of the positive rays in helium, while the middle points of the sides *• Goldstein, " Phil. Mag." VI, p. 372,1908.**