In the analysis of crystal structures by means of x-rays it is well known that when the reflecting plane does not contain the axis of rotation of the crystal (in other words, when the diffraction spot does not lie on the equatorial line of the photograph), the angular velocity of the plane is effectively decreased. This results in reflection by such a plane during a longer interval of time in each rotation than by planes producing spots on the equatorial or zero layer-line. Cox and Shaw have shown that the effect is purely geometrical, depending only on the angle of reflection and the orientation of the reflecting plane with respect to the axis of rotation of the crystal, and they have calculated the magnitude of this effect for rotation photographs taken with the x-ray beam perpendicular to the rotation axis of the crystal. In view of the fact that it is advantageous to take Weissenberg layer-line photographs with the rotation axis of the crystal inclined to the incident x-ray beam at an angle equal to that made by the rotation axis with the reflected x-ray beams for the layer-line that is being analyzed, the author has derived the relationship for the rotation factor De appropriate under such conditions. As Cox and Shaw have stated, the intensity of the spot produced by a reflecting plane is inversely proportional to ω/ Ω, where ω is the angular velocity of the crystal about an axis perpendicular to the plane containing the incident and reflected x-ray beams, and Ω is the angular velocity of the crystal about its actual rotation axis.

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