F1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.36192\,\lambda^2}{\lambda^2 - 0.0102049} + \frac{0.209163\,\lambda^2}{\lambda^2 - 0.048029} + \frac{0.907277\,\lambda^2}{\lambda^2 - 109.212}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.04 \times 10^{-7} + 2(1.92 \times 10^{-8})\,\Delta T + 3(-2.95 \times 10^{-12})\,\Delta T^2 + \frac{1.05 \times 10^{-6} + 2(4.87 \times 10^{-10})\,\Delta T}{\lambda - 0.23}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$