  Precise measurement of the refractive index of stable silicon isotopes
$^{28}$Si, $^{29}$Si, $^{30}$Si single crystals with enrichments above 99.9
at.% and a silicon single crystal $^{nat}$Si of natural isotopic composition is
performed with the Fourier-transform interference refractometry method from
1.06 to more than 80 mkm with 0.1 cm$^{-1}$ resolution and accuracy of $2
\times 10^{-5} ... 1 \times 10^{-4}$. The oxygen and carbon concentrations in
all crystals are within $5 \times 10^{15}$ cm$^{-3}$ and the content of metal
impurities is $10^{-5} ... 10^{-6}$ at.%. The peculiar changes of the
refractive index in the phonon absorption region of all silicon single crystals
are shown. The coefficients of generalized Cauchy dispersion function
approximating the experimental refractive index values all over the measuring
range are given. The transmission and Raman spectra are also studied.
