High-purity silicon is a readily available material of utility in realizing a variety of long-wavelength optical and guided wave components. The transmittance of uncompensated for silicon is measured in the far- and mid-infrared regimes at room and cryogenic temperatures. The experimental and analysis techniques used to extract the refractive index from 100-1000cm-1 (100-10 µm) are presented, and the results are compared to the literature. An average refractive index below 300cm-1, n^(300K)=3.417+i8.9×10-5, which transitions in cooling to n^(10K)=3.389+i4.9×10-6, is observed.