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IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
cited within the lead box in which the crystal and photographic 
plate are placed. The time of exposure may be made much less 
by first determining the approximate positions of these lines by 
taking a photograph using a wide slit as the source, then for 
the final photograph the crystal is rotated just through the 
exact angle required to cover the desired region. 
The method used to determine the exact angular position was 
as follows : A crystal rock salt was cemented to the lower 
half of a piece of plate glass and was then ground off until only 
a thickness of 0.2 millimeter remained. The upper half of the 
glass was silvered. Thus when mounted on the rotating axis 
the upper half would reflect a beam of light which would be 
exactly above the position at which the X-ray reflected by the 
crystal on the lower half would strike. 
The accompanying photograph was taken while slowly rotat- 
ing the crystal from 1° 30' to 2° 30' on each side of the center 
line. The total time required was 40 hours and the applied 
potential was between 80,000 and 100,000 volts. The distance 
of the crystal from the plate was 60.896 cm. and the distance of 
the crystal from the source was approximately the same. By 
measurement of the photograph the distance between the two 
outer lines is found to be 9.220 cm. Dividing this by 2 gives 
4.610 cm. as the amount of deviation from the center. Then 
4.610 divided by 60.896 is the tangent of twice the glancing 
angle of reflection which is thus found to be 4° 19.74'. Hence 
the glancing angle is 2° 9.87' 
The wave length is then given by the formula x =2d sin © in 
which d is the distance between planes of atoms of which the 
generally accepted value for rock salt is 2.814xl0- 8 cm. as given 
by Bragg. The wave length is then 2x2. 8 14x1 0- 8 x sin 2° 
9.87'=0.2126xl0- 8 cm. 
Figuring the other lines in the same way the characteristic 
K radiation of tungsten is found to be reflected at the following 
angles from rock salt and to have the following wave lengths. 
Angle 
Wave Length 
2° 9.87' 
0.2126x10-® cm. 
2° 6.93' 
0.2078 
1° 52.18' 
0.1837 
1° 49.06' 
0.1785 
Physics Laboratory, 
The State University. 
