ON THE SPECTRUM OF HYDROGEN. 
377 
pressure at wliich the walls of the vacuum tube show a vivid green fluorescence and 
the intrinsic intensity of the light is greatly reduced. 
It has long been known that when powerful condensed discharges are passed through 
vacuum tubes containing Hydrogen, the secondary spectrum disappears and the lines of 
the Balmer series alone remain, but we have observed that with a condenser and a 
rather small spark-gap in the circuit a group of lines extending from 26000 to 25600 
are very prominent; these lines are intrinsically weakened by the inclusion of the 
condenser and spark-gap in the circuit, but they are strong in comparison with the 
remaining lines of the secondary spectrum. If the length of the spark-gap is adjusted 
carefully, the effect is very striking if the spectrum is examined with a small direct vision 
instrument of low dispersion. These changes in intensity are given in the column headed 
“ Condensed Discharge.” 
Under “ Helium Effect ” are shown the changes of relative intensity which take place 
when Helium, at pressures up to 40 mm. and more, is admitted to the discharge tube 
with the Hydrogen. The phenomena which occur have already been described by one 
of us (T.'R.M.) ( loc. cit.). but in the present investigation we have extended this classi¬ 
fication of the lines into the more refrangible portions of the spectrum by adopting a 
suitable standard of intensity in the comparison-plates. 
We have confirmed the previous observation, in the presence of Helium, of a number 
of lines which do not appear under ordinary conditions, and the wave-lengths of these 
lines in the tables are given in brackets. 
In the next column are given the results obtained by Dufour (loc. cit.) in his investi¬ 
gations of the Zeeman effect; all the lines examined by Dufour have been marked 
O or Z in the tables, according to whether they show or do not show the Zeeman 
effect. 
The lines which have been arranged by Fulcher into bands have been noted in the 
succeeding column, and in the last column the results obtained by Takamine and 
Yoshida (loc. cit.), Nitta (loc. cit.) and Takamine and Kokubu (loc. cit.) in their 
investigations of the Stark effect are given. 
These different methods of classification are related to one another, but there are 
numerous exceptions to any broad generalization. In the red and yellow regions all 
the lines which were found by Dufour to show the Zeeman effect are “ High Pressure ” 
lines, and most of them are strengthened in the condensed discharge ; many of these 
lines also are enhanced by the presence of Helium, but there are exceptions to this 
rule. 
The Fulcher band lines are essentially “ Low Pressure ” lines, and are weakened in 
the condensed discharge. 
The exceptions to this rule are as follows :— 
26197-05 high pressure line. 
26093-83 somewhat enhanced in condensed discharge. 
25989-22 high pressure line, enhanced by condensed discharge. 
VOL. CCXXII.—A. 3 G 
