INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
143 
to the focal lengths of the lenses. With this arrangement, an exposure was made 
for a definite time on the plate. 
A perforated metal plate was then introduced between the two lenses, and another 
exposure for the same period of time was made on an adjacent portion of the same 
plate. This perforated plate consisted of a thin sheet of metal drilled with small 
holes at regular intervals of about a millimetre. By taking the mean of a number of 
micrometric determinations of the diameters of these holes and of the distances 
between them, the effective “ density ” of the metal plate could be calculated. The 
difference in the lengths of corresponding lines in the two spectra thus denotes the 
density step due to the plate, which is equal to the “ density ” of the perforated 
metal plate, from which the density step per millimetre length of wedge was 
calculated. The values thus found by the use of all the stronger Helium lines were 
plotted on squared paper against the wave-lengths, and a curve was drawn through 
these points. This curve was quite regular, and of the same type as that shown in 
the previous paper for another wedge, though obtained now by a different and in 
some respects better method. Actual values of the density gradient may be found 
in the tables given in later sections of this communication. 
(IV.) Experimental. 
In the present investigation we have examined the radiation in front of a flat 
aluminium cathode about 1 inch in diameter, which fitted closely into a cylindrical 
tube, as in fig. 1. The tubes were highly exhausted by means of a Gaede mercury 
pump, and after continuous sparking, connection with the pump was cut off and 
Helium was introduced by heating a quantity of powdered Thorianite contained in a 
fused silica bulb, which was connected with the vacuum tubes through a tube 
containing pieces of caustic potash and a U-tube containing charcoal cooled with 
liquid air. After sparking for some time, the tubes were sealed off, and were found 
to contain, in addition to the Helium, a certain amount of Hydrogen and also of 
Mercury vapour. A great part of the latter disappeared on further sparking, and 
finally the Mercury spectrum settled down to a constant intensity. The pressure in 
the tubes was such that the thickness of the dark space was about 1 mm. With 
electrodes of these dimensions, the tubes could be run with a moderate current for an 
