104, Mr Watson, Some EKxperiments on the 
a lamp. After five or six hours the glow became far paler, 
and it was found that if the current were switched off and 
then on again, the lamp would not light up, although it 
would do so if the current were reversed. This seems to be 
an effect similar to that in connexion with the initial spark 
potential. It occurred in the case of both electrodes, and can 
hardly be ascribed to true fatigue, since, if the current passed 
only for a small fraction of a second, the tube would not again 
light up. Reversal of the current, and possibly alteration of 
pressure, seems to produce some molecular rearrangement of the 
surface of the cathode. 
This tube also showed signs of true fatigue, and after about 
60 hours would not light at all with the potential difference in 
question. 
It will thus be seen that with helium and neon there are 
considerable fatigue effects similar to the well known phenomenon 
of photo-electric fatigue, and probably arising from the same 
cause. 
Intensity of the Illumination. 
Anyone who has seen a discharge through pure neon will be 
aware of the intense brilliancy which characterises it. The 
brightness of the negative glow on a flat electrode at pressures in 
the neighbourhood of 10 mm. is remarkable, and it was thought 
that the efficiency of this light might be very high, and more- 
over, as it appeared likely before starting the experiments that 
the potential difference necessary to start and maintain a current 
would be very low, it seemed not improbable that it would be 
possible to make an efficient lamp which would run from an 
ordinary lighting circuit. Nearly ail the experiments so far 
described were carried out with this end in view, and this accounts 
for any apparent lack of connection between them, and in many 
cases their incompleteness. 
A number of very rough measurements of intensity were 
made with a grease spot photometer, the voltage and current 
being measured simultaneously, but it is not proposed to give 
these in detail. One very interesting fact was that the intensity 
of the light when measured was found to be far less than when 
estimated. Some tubes which were dazzlingly bright were of con- 
siderably less than one candle power, and the estimated efficiency 
was correspondingly reduced. 
There appeared in all cases to be a maximum efficiency for 
medium currents, which were of the order of 30 milliamperes for 
the small tubes already described. The pressure exerted a great 
influence, for example, with copper electrodes, at 3 mm. pressure 
