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20 RAYS OF POSITIVE ELECTRICITY

the tube is big the walls are far away from the cathode and
the pressure has to be exceedingly low before the dark space
reaches the sides of the tube. We can work with much lower
pressures with these large tubes and therefore reduce the ob-
struction which the positive rays meet with in their passage
from the cathode to the screen. Using vessels of about 2
litres capacity I have observed¹ on a willemite screen the
parabolas corresponding to carbon, oxygen, neon, and mercury
vapour as well as those corresponding to the atom and
molecule of hydrogen and the atom of helium. The photo-
graphic plate is, however, for most purposes a much more

FIG. 10.

convenient detector than a willemite screen. It is more sensi-
tive, it gives a permanent record, and measurements can be
made with much greater accuracy on the plate than they can
oil the screen. Before entering into the discussion of the
theory of the positive rays it is desirable to describe the
results obtained with the photographic method, as well as the
experimental details by which these results have been pro-
cured.

The apparatus now in use at the Cavendish Laboratory is
represented in Fig. 10. The discharge takes place in a large

¹J. J. Thomson, " PML Mag.," VI, xx, p. 752, 1910,



	20 RAYS OF POSITIVE ELECTRICITY the tube is big the walls are far away from the cathode and the pressure has to be exceedingly low before the dark space reaches the sides of the tube. We can work with much lower pressures with these large tubes and therefore reduce the ob- struction which the positive rays meet with in their passage from the cathode to the screen. Using vessels of about 2 litres capacity I have observed¹ on a willemite screen the parabolas corresponding to carbon, oxygen, neon, and mercury vapour as well as those corresponding to the atom and molecule of hydrogen and the atom of helium. The photo- graphic plate is, however, for most purposes a much more



	FIG. 10. convenient detector than a willemite screen. It is more sensi- tive, it gives a permanent record, and measurements can be made with much greater accuracy on the plate than they can oil the screen. Before entering into the discussion of the theory of the positive rays it is desirable to describe the results obtained with the photographic method, as well as the experimental details by which these results have been pro- cured. The apparatus now in use at the Cavendish Laboratory is represented in Fig. 10. The discharge takes place in a large ¹J. J. Thomson, " PML Mag.," VI, xx, p. 752, 1910,