Unusual Reflecting Power of a Pair of Calcite Crystals

Reflection characteristics of the calcite (Iceland spar) crystal are reported. Two split calcite crystals were mounted on double x-ray spectrometer in parallel position. Coefficient of reflection, percent reflection and width of rocking curve were measured at both first and second order reflections and for three wave-lengths: $\lambda 0.209\mathrm{A}$, $\lambda 0.7077\mathrm{A}$ and $\lambda 1.537\mathrm{A}$. The percent reflection was as great as 48.5 at first order for $\lambda 1.537$. The corresponding coefficient of reflection was small. The rocking curves were surprisingly narrow, being as small as 1.25″ at second order for $\lambda 0.7078\mathrm{A}$. This width is less than is to be expected from the Darwin equation using previously determined values of the structure factor. Crystals of these properties are especially valuable for use on the double crystal spectrometer, when high resolving power is desired.