PHYSICS: C. D. MILLER 
443 
then 
Ve = 
(2) 
(3) 
The mass per square centimeter of aluminum, pd, which was found 
necessary to render the X rays approximately homogeneous, is given, 
as nearly as could be observed, by 
pd = 10-12 X 
(4) 
It has been known for some time that an equation of the form of 
(3) would represent roughly the relationship between absorption coef- 
ficients and wave lengths, but the exponent has been estimated by 
Alrsarlrtion CMffet-ienti, 
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»nd tkc thKiritMS tra.*erie< 
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the mii.d.i* of tk# tlieet Irf 
wkuh the correthorliM win* 
0. M 
mo- 
an 
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0 
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1 
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7a a 
Bragg,^ who is perhaps the foremost observer, at from 2.5 to 3.0, whereas 
these results indicate it to be 2.77 ± 0.03. Bragg's estimate is based on 
the absorption coefficients of monochromatic rays obtained by the use 
of the X ray spectrometer. It has also been known^ that X rays pro- 
duced at steady potentials become homogeneous after passage through 
a sufficient thickness of absorbing material. However, no formula 
such as (4) has been given by which to estimate the necessary thick- 
ness. Data similar to those on which equation (1) is based have 
been used by Duane and Hunt^ and by Rutherford, Barnes and Richard- 
son^ to calculate wave lengths by means of relationships similar to (3), 
