F13 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.35317\,\lambda^2}{\lambda^2 - 0.0100821} + \frac{0.207757\,\lambda^2}{\lambda^2 - 0.0477132} + \frac{0.90216\,\lambda^2}{\lambda^2 - 109.427}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-9.71 \times 10^{-7} + 2(6.95 \times 10^{-9})\,\Delta T + 3(-1.86 \times 10^{-11})\,\Delta T^2 + \frac{1.37 \times 10^{-6} + 2(1.47 \times 10^{-9})\,\Delta T}{\lambda - 0.132}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$