The (220) lattice plane spacing of an almost perfect crystal of silicon was measured by means of a combined scanning ($\mathrm{LLL}$) x-ray interferometer and a two-beam optical interferometer. From 170 measurements, a value $d_{220}=(192\mathrm{}015.560\pm 0.012)$ fm results in vacuum at 22.50°C. This value is smaller by $1.8\times {10}^{-6\mathrm{}}{d}_{220}$ than that reported by Deslattes et al. for another crystal. Generic variabilities of the two crystals account only for a part of this difference.

• Received 3 February 1981

DOI:https://doi.org/10.1103/PhysRevLett.46.1540