92 
PH YSICS: CLA RK A ND D UA NE 
Proc. N. a. S. 
periment performed with the crystal turned through 90 ° indicates that KI 
has a cubic lattice, which agrees with previous work by Davey and Wyckoff . 
The accuracy with which the experimentally determined angles agree 
with those given by solid geometry amounts in reality to a test of the 
precision with which the instrument has been set up, i.e., so that an axis 
of the crystal lies parallel to the axis of rotation, and so that the center 
of the incident beam of rays and the effective reflecting plane of the crystal 
lie on the axis of rotation. 
One peak of the curve, marked A', does not correspond to any of the 
chief planes of the crystal, i.e., those with small indices. Its position 
shows that the corresponding planes make an angle of 17°.84 with the 100 
planes. Possibly the marked reflection at this angle may be due to the 
characteristic line spectrum of iodine. Further evidence on this point 
will be given in a later paragraph. 
The wave-lengths, X, of the X-rays reflected at the angle B, corre- 
sponding to any one of the peaks, are given by the fundamental law of 
X-ray reflection, which may be expressed by the equation 
n\ = 2iisin^, (1) 
where n is a whole number, and d is the distance between successive planes. 
We cannot use this equation alone to determine d, for we do not yet know 
the value of X. The peaks on the curve in figure 1 give us the angles be- 
tween various planes only. 
In order to determine the distance, (i, we make use of the fact^ that, 
in order to produce an X-ray of wave-length 7, the voltage, V, applied 
to the X-ray tube must have a value at least as large as that given by the 
equation 
he 
Ve = (2) 
where h is Planck's radiation constant, e the charge of an electron (e) and 
c the velocity of light. This equation ma}^ be written in the form 
he 
V\ = — = 12,354, (20 
e 
where the numerical value^ of the universal constant, hc/e, corresponds to 
V expressed in volts and 7 in Angstroms. 
In an actual experiment to measure d we set the crystal so that the X- 
rays reflected by the planes we desire to investigate enter the ionization 
chamber (i.e., we set on one of the peaks in the curve of figure 1), and re- 
duce the voltage until the ionization current sensibly vanishes. The 
voltage at which it vanishes is sharply marked, especially if the voltage 
is a constant one, from a storage battery, for instance. It is a good plan 
to increase the current through the tube at the same time as the voltage 
decreases in such a way as to keep the product of the two, that is, the 
