Index of Refraction of Calcite for X-rays

Index of refraction of calcite for $\mathrm{Mo}\mathrm{K}{\alpha }_1$ rays.—Previous work has indicated that in the case of first order reflection from calcite the bending due to refraction is only 3″. As suggested by Bergen Davis, however, the effect can be greatly increased by using a wedge shaped crystal with a surface ground and polished so as to make an angle $\varphi$ with the cleavage planes slightly less than the angle of reflection. The first crystal was ground with $\varphi$ equal to 5°48.3′ so that rays which entered at an angle of 12.5° with the surface, after first order reflection left making an angle of 54′ with the surface. The refraction bending was 29″. A second crystal was ground with $\varphi =6^{\circ }{21.5}^{\prime }$ so that the glancing angle was only 21′ and the refraction bending was 64″. The lack of symmetry due to the refraction bending at one surface was determined either by rotating the crystal about a horizontal axis perpendicular to the crystal planes or by using two halves of a split crystal, one ground and the other with natural cleavage, placed one over the other so as to get reflection from each in turn. The mean result is $\mu =1-(2.03\mathrm{}\pm .1)\times {10}^{-6\mathrm{}}$. This agrees well with the value given by the Lorentz dispersion formula, 1—1.91×${10}^{-6\mathrm{}}$.