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PHYSICS: D. L. WEBSTER 
and a doublet. The critical potential is very sharply defined, and has 
a value 23.3 kv, while the quantum potential, at which radiation of 
that wave length first begins to appear, is 20.05 kv. The same effect 
is shown by the ^ line, as one may see by comparison of its intensity- 
potential curve (fig. 1) with that of the general radiation of a neighbor- 
ing wave length. The effect is less pronounced because the line is weaker 
than a. The critical potential, as nearly as one can tell, is the same in 
both cases. 
Fig. 2 
To prove that the corner in the a curve really does mean the first ap- 
pearance of the a lines, photographs of this part of the spectrum have 
been taken at different potentials above and below the critical one. 
These show that the ratio of the intensity of the Hnes to that of the 
background, as well as the absolute intensity of either, increases rapidly 
with the potential, and that, while the lines are visible against the back- 
ground within 1% above the critical potential, they are absolutely invis- 
ible below it. 
The next point to consider is the ratio of intensities of any two lines 
and its dependence on potential. From numerous photographs with 
different exposures it appears that the ratio of ai and a2 is constant, as 
