SF9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.41374\,\lambda^2}{\lambda^2 - 0.0105642} + \frac{0.241836\,\lambda^2}{\lambda^2 - 0.0490126} + \frac{1.01292\,\lambda^2}{\lambda^2 - 117.884}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.97 \times 10^{-6} + 2(1.67 \times 10^{-8})\,\Delta T + 3(-6.4 \times 10^{-11})\,\Delta T^2 + \frac{1.04 \times 10^{-6} + 2(3.93 \times 10^{-10})\,\Delta T}{\lambda - 0.263}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$