INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
lof) 
Table XT.— Helium and Parlielium. 
Photo- 
Photo- 
Photo- 
Photo- 
Photo- 
Photo- 
Photo- 
Photo- 
Series. 
/V. 
graph 
graph 
graph 
graph 
grapli 
grapli 
grapli 
grajih 
1 . 
H. 
HI. 
IV. 
V. 
VI. 
VH. 
VIH. 
Diffuse . . . < 
■ 
.5876 
6678 
10 
4 • 22 
10 
4'81 
10 
3 • 80 
10 
2-98 
10 
2-41 
10 
2 • 25 
10 
2-05 
10 
Sharp. . . .< 
r 
4713 
10 
10 
10 
10 
10 
10 
_ 
— 
5047 
1-75 
1-33 
— 
— 
— 
— 
— 
Principal. 
r 
3888 
10 
10 
10 
10 
10 
10 
10 
— 
5015 
7-59 
9-00 
7-69 
6-74 
4-68 
6-18 
5-12 
— 
of the dark space is shown definitely also in the Principal series of Parhelium, which 
at this point becomes almost as intense as that of Helium. Subsequently the decrease 
of the Parhelium spectrum is shown definitely until we arrive at VI., where another 
temporary reversal occurs. Although the photographs VI.-VHI. may not be strictly 
comparable with the others, the phenomenon appears to be real, for it corresponds to 
similar effects in previous comparisons of series made in this communication, which 
are not restricted to special ranges of wave-length in which the plate on which 
VI-VIII. were taken might have special properties. Moreover, it does not occur at 
all in other series, for example the two Diffuse series of the present section, where 
there is no sudden change in the character of the numbers characterizing X6678 on 
passing from V.-VI. 
It seems necessary to conclude that there is a region, distant about 5 mm. from the 
cathode in the present experiment, where, as at the extremity of the dark space— 
1 mm. from the cathode—the conditions of excitation reach some form of critical 
point, with a consequent change in the nature of the law of intensity variation of 
certain lines and series with cathode distance. In particular, there is a tendency for 
relative enhancement of the Principal series of Parhelium, but not the Diffuse series, 
at this point. 
(VHI.) Regions of Maximum, Emission. 
The regions in the tube from which individual spectral lines are radiated with 
greatest intensity are of considerable importance in connection with theories of the 
origin of spectra. The present measurements ejiable us to obtain some quantitative 
data with regard to many lines in the spectrum of Helium. We do not attempt to 
discuss all the lines from this point of view, the exposure being in many cases only 
sufficient to show some of the lines on one or two photographs, so that no graphical or 
other method can be used to determine the exact law which connects their intensities 
with the distances from the cathode, enabling tbe positions of the maxima to be read 
off the curve of intensity or calculated by analysis. Moreover, it is sufficient, for a 
z 2 
