F6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.3864\,\lambda^2}{\lambda^2 - 0.010312} + \frac{0.216317\,\lambda^2}{\lambda^2 - 0.0482392} + \frac{0.920243\,\lambda^2}{\lambda^2 - 111.029}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.99 \times 10^{-7} + 2(1.59 \times 10^{-8})\,\Delta T + 3(-2.94 \times 10^{-11})\,\Delta T^2 + \frac{9.98 \times 10^{-7} + 2(1.01 \times 10^{-9})\,\Delta T}{\lambda - 0.245}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$