F8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31172\,\lambda^2}{\lambda^2 - 0.00965178} + \frac{0.17248\,\lambda^2}{\lambda^2 - 0.0461712} + \frac{0.840159\,\lambda^2}{\lambda^2 - 100.047}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.4 \times 10^{-6} + 2(1.54 \times 10^{-8})\,\Delta T + 3(-2.78 \times 10^{-11})\,\Delta T^2 + \frac{8.83 \times 10^{-7} + 2(9.33 \times 10^{-10})\,\Delta T}{\lambda - 0.234}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$