F15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31032\,\lambda^2}{\lambda^2 - 0.00954035} + \frac{0.202637\,\lambda^2}{\lambda^2 - 0.0455807} + \frac{0.920049\,\lambda^2}{\lambda^2 - 109.865}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.95 \times 10^{-6} + 2(1.6 \times 10^{-8})\,\Delta T + 3(-2.68 \times 10^{-11})\,\Delta T^2 + \frac{8.66 \times 10^{-7} + 2(8.75 \times 10^{-10})\,\Delta T}{\lambda - 0.248}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$