Spectrum Optical constants of Mg$_{0.1}$Fe$_{0.9}$S in MIR/FIR Spectrum Optical constants of Mg$_{0.5}$Fe$_{0.5}$S in MIR/FIR Spectrum Optical constants of Mg$_{0.75}$Fe$_{0.25}$S in MIR/FIR Spectrum Optical constants of Mg$_{0.9}$Fe$_{0.1}$S in MIR/FIR Spectrum Optical constants of FeS in MIR/FIR