F9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.37298\,\lambda^2}{\lambda^2 - 0.00970907} + \frac{0.186344\,\lambda^2}{\lambda^2 - 0.0477952} + \frac{0.996847\,\lambda^2}{\lambda^2 - 109.902}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.21 \times 10^{-6} + 2(1.51 \times 10^{-8})\,\Delta T + 3(-2.72 \times 10^{-11})\,\Delta T^2 + \frac{8.99 \times 10^{-7} + 2(1.23 \times 10^{-9})\,\Delta T}{\lambda - 0.245}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$