Vol. 6, 1920 PHYSICS: DUANE, FRICKE AND STENSTROM 
611 
TabIvE I. 
K CRITICAL ABSORPTION WAVE-LENGTHS, XXIO^ Cm. 
For Calcite X = 6.056 Xs in ^XlO-^Cm. 
CHEMICAL 
ELEMENT 
Tungsten' 
Platinum. 
Gold. . . . 
Mercury . 
Thallium. 
Lead. . . . . 
Bismuth . 
Thorium . 
Uranium . 
74 
78 
79 
80 
81 
82 
83 
90 
92 
152 
149 
146 
142 
138 
133 
.1578 
.1524 
.1479 
.1427 
.1385 
.1346 
.1127 
.1048 
AUTHORS VALUES 
. 1783 
.1582 
.1537 
.1493 
.1448 
.1412 
.1375 
1781 
.1409 
.1369 
.1139 I .1124 
.1075 
.1780 
.1581 
.1532 
.1488 
.1449 
1781 
1581 
1534 
1491 
1448 
1410 
1372 
1131 
1075 
2.370 
2.515 
2 . 553 
2.590 
2.628 
2.663 
2.700 
2.974 
3.051 
* The wave-lengths for tungsten were measured in cooperation with Dr. R. A. 
Patterson. 
column 10, the difference, between these square roots calculated per unit 
increase in atomic number. 
Discussion of the Results. — The differences tabulated in column 10 do 
not vary from their mean value more than one would expect from the 
experimental errors. This indicates that within the limits of error Vi/X 
is a uniformly increasing function of the atomic number. The differences, 
however, are slightly larger than the corresponding differences between 
the square roots of the critical absorption wave-numbers measured in 
previous researches^'* for chemical elements of lower atomic numbers. 
This means that the square root of the critical absorption wave-number 
is not quite a linear function of the atomic number, which agrees with 
the results noted in the earlier papers.*'''* The variation from the linear 
law, however, is too small to be clearly indicated in the range of values 
contained in table 1. 
The wave-lengths measured in spectra of the second and third order 
(columns 6 and 7) appear to be, with one exception, smaller than the 
corresponding wave-lengths measured in the first order spectrum. The 
differences between the two sets of values about equal the errors of meas- 
urement. Similar differences between spectra of the various orders were 
observed first by Dr. Stenstrom^ in his measurements of long X-rays by 
means of a photographic spectrometer. He attributed the effect to a 
slight refraction and dispersion of the X-rays by the refiecting crystal. 
Our measurements in the second order spectrum for thorium were not 
