37G 
DR. T. R. MERTON AND MR. S. BARRATT 
part in the red and yellow, and particularly in the green regions, in which it would 
have been impossible to record so many faint lines without the use of the Iso-RecOrd 
plates. It is believed that lines due to impurities have been excluded, all the photo¬ 
graphs used for measurement having been taken with tubes which had been run for 
some hours, and in which the discharge appeared to be white throughout the capillary. 
(4) The Continuous Spectrum. 
Tn addition to the Balmer series and the secondary spectrum, vacuum tubes con¬ 
taining Hydrogen always emit a continuous spectrum. The intensity and extent of 
this continuous background depends greatly on the pressure of the Hydrogen in the 
discharge tube. If the pressure is reduced to the stage at which the glass begins to 
show a green fluorescence, the continuous spectrum almost disappears and the secondary 
lines are sharp against a clear background, even in the ultra-violet. At about 50 mm. 
pressure the continuous spectrum extends into the green, and at still higher pressures 
it covers the whole spectrum. The introduction of capacity into the discharge circuit 
has no very marked effect on the continuous spectrum, but appears to weaken it to 
some extent, an effect which can also be produced by an admixture of Helium. Accord¬ 
ing to Lyman (‘ Spectroscopy of the Extreme Ultra-violet ’), the continuous spectrum 
fills the gap between the end of the secondary spectrum and the Hydrogen lines in the 
Schumann region, but the intensity distribution in this spectrum has not been studied. 
(5) The Classification of the Lines. 
In the tables of wave-lengths, under the heading “ Intensity ” will be found the 
estimated intensities of the lines, on the usual scale of 0 to 10, when the discharge tube 
containing Hydrogen at a pressure of a few millimetres was excited by an uncondensed 
discharge. Under these conditions the discharge is at its brightest, being intrinsically 
_ weaker both at higher and at lower pressures. In the column succeeding those in which 
the intensities are given, the effects of changes of pressure and other conditions on the 
relative intensities of the lines are shown. In all such cases the intensity changes have 
been estimated by examining a standard plate in juxtaposition with a plate taken under 
the conditions in question and exposed for a tune appropriate to the intrinsic brightness 
of the source. A + denotes that the line is enhanced and that it is greatly en¬ 
hanced, — and = denoting in the same way that the line is somewhat or greatly 
weakened as the case may be. The observations referred to under “High Pressure ” 
were made with tubes containing Hydrogen at pressures greater than 50 mm. of mer¬ 
cury, and under these conditions the discharge was much less luminous and of a bluish- 
white colour, the spectrum lines being superposed on a rather strong continuous back¬ 
ground. The changes which were found in the relative intensities of the lines are shown 
in the tables under the column “ High Pressure,” and in the succeeding column are 
given the changes observed at low pressures. By “ Low Pressure” we refer to a 
