Further details are given of an experiment based on combined X-ray and
optical interferometry to measure the (220) lattice spacing of silico
n. A resolution of 5 x 10(-9)d(220) was achieved and the silicon d(220
) was determined to 3 x 10(-8)d(220) accuracy. The measured value is d
(220) = (192015.551 +/- 0.005) fm. After correction for the impurity-i
nduced lattice strain, d(220) = (192015.569 +/- 0.006) fm was obtained
.