FK5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.844309\,\lambda^2}{\lambda^2 - 0.00475112} + \frac{0.344148\,\lambda^2}{\lambda^2 - 0.0149815} + \frac{0.91079\,\lambda^2}{\lambda^2 - 97.86}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-7.24 \times 10^{-6} + 2(1.58 \times 10^{-8})\,\Delta T + 3(-9.51 \times 10^{-12})\,\Delta T^2 + \frac{3.51 \times 10^{-7} + 2(4.61 \times 10^{-10})\,\Delta T}{\lambda - 0.156}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$