106 Mr Watson, Some Eaperiments on the 
individual. In consequence, during the whole of the experiments 
it was necessary to wear very dark brown glasses. 
The same effects have been noticed with argon, krypton and 
xenon although the light emitted was of the faintest kind. The 
most obvious “suggestion is that ultra-violet light is the cause, but 
an examination of the spectra of these gases shows that the 
intensity of the ultra-violet spectrum as measured photographi- 
cally is certainly less in all cases than that of the visible spectrum, 
and indeed, in krypton, there are practically no bright ultra= 
violet lines. i 
A property common to these gases is that their spectra all 
consist of a comparatively small number of intensely bright lines, 
and it would be interesting to know if these could possibly atfect 
the eye to the same extent as a continuous spectrum emitting for 
instance the same amount of energy per ;/,th A.U. as is given Y oul 
by one line of the gas spectrum. The total energy of the con-— 
tinuous spectrum would naturally be far greater, but for certain : 
colour senses the two would be equal, and an equal strain on — 
some portion of the eye might result. ; 
In conclusion, it may be mentioned that of the several points 
which have been dealt with in this paper, there is not one, our 
knowledge of which is in the least degree complete, and a further _ 
investigation would certainly in every case be amply repaid. 
I should like to take this opportunity of conveying my very 
best thanks to Sir J. J. Thomson in whose laboratory these | 
experiments were carried out, for the great interest he has at all 
times shown in them. 
SUMMARY. 
1. Measurements have been made of the spark potentials at 
different pressures, between plane aluminium electrodes in very 
pure helium and neon, no external exciting agent being used. 
2. In the case of electrodes 15 mm. apart, the normal 
minimum spark potentials excluding a lower value obtained on 
the first passage of the current, were found to be 184 and 
200 volts for helium and neon respectively, the corresponding 
pressures being 2°4 and 2°8 mm. respectively. For a perfectly 
uniform field the potential differences might possibly be two to 
three volts less. 
3. At pressures higher than those given above, the curve con- 
necting spark potential and pressure did not assume a linear form 
until the pressure rose to about 10 mm. After this point, the 
gradients were 30 volts per cm. pressure for helium, and 22 volts 
per cm. for neon (electrodes 15 mm. apart). 
