94 
PH YSICS: CLA RK A ND D UA NE Proc. N. A. S. 
second is 1.417, which differs Httle from V2 = 1.414, the value it should 
have in a cubic crystal. Taking 3.532 as the average length of the edge 
of a cube, and assuming one atom to each cube the calculated density of 
KI comes out 3.111, in close agreement with the density 3.114 measured 
by Baxter. 
he 
Equation (3) contains a universal constant ■ — = 12,354 and two 
c 
quantities, a voltage, V, and an angle, 6, both of which we measure. 
The values of d, therefore, determined by this method are independent 
of previous measurements of the distance between the planes in any 
crystal. This advantage is, perhaps, more theoretical than practical at 
present, for measurements with X-rays reflected from calcite probably 
furnish us the most accurate estimates we have for the universal constant. 
Kilovoits 
FIGURE 3 
The agreement between the distances, J, determined by this method with 
those calculated from the density, atomic weights, etc., helps to confirm 
the general correctness of the laws of X-rays as at present understood. 
If the specimen to be examined has the form of powder, we place the 
powder at the center of the spectrometer, fix the position of the ionization 
chamber at an angle 2d and reduce the voltage by steps making measure- 
ments of the ionization current as before. The ionization current vanishes 
when the voltage is no longer able to produce the longest X-rays that are 
reflected at the angle d by the powder. This maximum wave-length 
corresponds to w = 1 in equation (1) and to the greatest distance d be- 
