6io PHYSICS: DUANE, FRICKE AND STENSTROM Proc. N. A. S. 
FIG. 2 
more difficult to estimate their centers. The value of 6, however, is two 
or three times, as large as in the first order. 
Columns 5, 6 and 7 in table 1 contain the critical absorption wave- 
lengths estimated from the curves of figures 1 and 2 (and similar curves 
for the other chemical elements) in the spectra of the first, second and 
third orders respectively. Column 8 contains weighted mean values of 
the wave-lengths, column 9, the square roots of the wave-numbers, and 
