The calculation of the total-to-selective ratio R _V in the range of ﹩{a}_{\max }=[5\,\mathrm{nm}-0.5\,\mu {\rm{m}}]﹩ of three configurations of dust: astrosilicate (blue); carbonaceous (orange); and a mixture of both dust materials (green). In the latter case, we assume a ratio between astrosilicate and carbonaceous components of C _sil/C _carb = 1.12.