The refractivity of wild-type bacteriorhodopsin ( ${\mathrm{bR}}_{\mathrm{WT}}$ ) suspended in tris(hydroxymethyl)aminomethane (TRIS) buffer has been measured in the spectral range of $390–840\mathrm{nm}$ by the method of angle of minimal deviation with the use of a hollow glass prism. The refractive indices of pure ${\mathrm{bR}}_{\mathrm{WT}}$ as well as of TRIS buffer have been determined from the concentration dependent refraction values. Sellmeier-type dispersion equations have been fitted for both the TRIS buffer and pure ${\mathrm{bR}}_{\mathrm{WT}}$ .