240 PHYSICS: DUANE, PALMER AND YEH Proc. N. A. S. 
of the focal spot as seen from the spectrometer's slit, and then measure the 
width of the slit by the method described on p. 630 of the paper by Blake 
and Duane referred to above. 3 Secondly, we measure the breadth of the 
drop in the ionization curve due to the K critical ionization of bromine. 
The ionization chamber containes ethylbromide. Curve C in the figure 
^presents this drop in the curve in one of the experiments. The breadth 
of this drop corresponds within the limits of error of the measurements to 
the correction to be added to the double angle 20, as determined by the 
first method. The correction is small. It amounts to less than one part 
in three hundred. 
Incidentally, the measurements we have made of the critical ionization 
of bromine gives a very accurate measurement of its critical ionization 
wave-length. As an average we obtained the value 
\ = (.9180 ± .0002) X 10- 8 cm., 
assuming that the grating constant of calcite is 6.056 X 10 ~ 8 cm. 
We tried to obtain an estimate of the correction for the breadth of the 
source and the slit by measuring the breadth of a peak on the ionization 
curve corresponding to a line in the characteristic emission spectrum of 
the tungsten target. In every case examined, however, the breadth of 
the peak appeared to be slightly broader than what the measured breadth 
of the source and slit indicated it should be. This means that the corre- 
sponding emission lines have certain finite, intrinsic breadths of their own. 
If the K critical ionization of bromine has such an intrinsic breadth it is 
too small to be detected in these experiments. 
Since from the wave equation, 
XXi> = c = 2.9986X10 10 
where c is the velocity of light, we can calculate immediately the maximum 
frequency v in the continuous spectrum. This, together with the difference 
of potential, V, substituted in equation 1, gives us the ratio of h to e. 
The data obtained in four complete measurements of h appear in table 
1. Column 2 contains the uncorrected values of 0, column 3 the correc- 
TABLE 1 
Glancing Angles and Radiation Constant 
DATE 
6 (uncorrected) . 
A9 
^(corrected) 
Vsind 
h 
March 15 
4°-46'-43" 
47" 
4°-47 , -30" 
2039.2 
6. 5539X10-" 
March 21 
4°-46'-53" 
43" 
4°-47 , -36" 
2039.9 
6.5561 " 
March 30 
4°-46'-53" 
51" 
4°-47 , -44" 
2041.0 
6.5594 " 
April 5 
4°-46'-48' / 
45" 
4°-47 / -33" 
2039.6 
6.5552 " 
Mean 
2040.0 
6. 5562 X 10 " 27 
tion for the breadths of the source and of the slit, and column 4, the corrected 
values of 0. Column 5 contains the values of the product V $md (which is 
what we really measure in our experiments) . This product depends to a 
slight extent upon the te mperature. The temperature in these experiments 
