In the double spectrometer used, x-radiation was reflected from a crystal surface $A$ through a slit $S$ on to a second crystal surface $B$, and thence to the ionization chamber. Surfaces $A$ and $B$ were obtained by splitting a single crystal of calcite. The curve obtained for Mo $\mathrm{K}\ensuremath{\alpha}$ by rocking $B$ is a triangle with base about 8 seconds wide. The effect of interposing a prism of aluminum (angle 166\ifmmode^\circ\else\textdegree\fi{}) between $S$ and $B$ was to shift the curve 5.62\ensuremath{''} on the average, giving $1\ensuremath{-}\ensuremath{\mu}=1.68\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. The value calculated from the Lorentz dispersion formula is 1.77\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. For Cu $\mathrm{K}\ensuremath{\alpha}$ the shift was 5.53\ensuremath{''} for a 116\ifmmode^\circ\else\textdegree\fi{} prism, giving $1\ensuremath{-}\ensuremath{\mu}=8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ as compared with the calculated value 8.36\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. The accuracy of this method is thus within 5 percent.