Soft-x-ray Bragg reflection from two $\mathrm{Ru}/{\mathrm{B}}_{\mathrm{4}}\mathrm{C}$ multilayers with 10 and 63 periods was used for inde pendent determination of both real and imaginary parts of the refractive index $n=1-\delta +i\beta$ close to the boron K edge ( $\sim 188\mathrm{eV}$ ). Prior to soft x-ray measurements, the structural parameters of the multilayers were determined by x-ray reflectometry using hard x rays. For the 63-period sample, the optical properties based on the predictions made for elemental boron major deviations were found close to the K edge of boron for the 10-period sample explained by chemical bonding of boron to ${\mathrm{B}}_4\mathrm{C}$ and various boron oxides.