F3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.32797\,\lambda^2}{\lambda^2 - 0.00977103} + \frac{0.205863\,\lambda^2}{\lambda^2 - 0.0463606} + \frac{0.904336\,\lambda^2}{\lambda^2 - 107.864}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.18 \times 10^{-6} + 2(1.65 \times 10^{-8})\,\Delta T + 3(-2.14 \times 10^{-11})\,\Delta T^2 + \frac{9.72 \times 10^{-7} + 2(8.88 \times 10^{-10})\,\Delta T}{\lambda - 0.235}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$