F7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.35919\,\lambda^2}{\lambda^2 - 0.0101796} + \frac{0.209887\,\lambda^2}{\lambda^2 - 0.0483606} + \frac{0.881406\,\lambda^2}{\lambda^2 - 110.387}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.69 \times 10^{-6} + 2(1.45 \times 10^{-8})\,\Delta T + 3(-2.52 \times 10^{-11})\,\Delta T^2 + \frac{9.15 \times 10^{-7} + 2(9.02 \times 10^{-10})\,\Delta T}{\lambda - 0.259}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$