F 
: Electrical Discharge in Helium and Neon. 105 
the efficiency was about 90 watts per candle, while at 12°7 mm. it 
was 20. A similar tube with aluminium electrodes at the former 
pressure had an efficiency of about 40 w.p.c. More satisfactory 
results were obtained with tube no. 2 with a sodium potassium 
electrode. Here the efficiency was increased to 4 w.p.c. for 
pressures between 10 and 16 mm., the voltage across the tube 
being 90, but this of course is far above the limit required for 
practical illumination. Experiments were started with a number 
‘of other metals as cathodes, but were unavoidably broken off. 
_ The above figures were only obtained with gas of the utmost 
purity free from mercury vapour, and after a short time, in spite 
of all precautions, it invariably became contaminated, and the 
brightness diminished. This was less noticeable when the cathode 
was of sodium potassium alloy owing to the absorption of the 
impurity, but this liquid is most inconvenient as it tends to soil 
the glass, and it is almost impossible to coat a metallic surface 
evenly with it. 
While these experiments were in progress, I was informed 
privately that similar work was being carried on by M. Claude 
in Paris. Some of his results have now been published (C. Rh. 
1910, 151, p. 1122; 1911, 152, p. 1377), and are much more 
satisfactory from a practical point of view, an efficiency of 0°64 
watts per candle having been attained. In his experiments no 
attempt was made to keep the voltage low, 1000 volts being used, 
and the light appears to be that from the positive column, 
although this is not precisely stated. In my experiments the 
light was derived from the negative glow, and the amount of 
energy lost in the dark space is many times the amount which 
produces luminosity. On the other hand, when high voltages are 
utilised, the dark space loss becomes negligible. 
Physiological Effects. 
The light given out by helium and neon under the influence 
of the electric discharge is of a peculiar nature. It has already 
been mentioned that the apparent brightness is far in excess of 
the actual intensity, but what is more remarkable is the acute 
physiological effect of such a feeble illuminant. In the foregoing 
experiments it was found that if the light from one of these 
tubes emitting say ;!,th of a candle power, was allowed to fall on 
the unprotected eyes for about a quarter of a minute, a violent 
headache was the result, followed by temporary blindness, while 
on one occasion, looking at a tube of one candle power for one or 
two seconds produced pain in the eyes, and a total inability to 
read for two days, and these effects were not confined to one 
