SF19 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.45496\,\lambda^2}{\lambda^2 - 0.0109533} + \frac{0.237838\,\lambda^2}{\lambda^2 - 0.0505231} + \frac{0.958586\,\lambda^2}{\lambda^2 - 109.77}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.21 \times 10^{-6} + 2(1.7 \times 10^{-8})\,\Delta T + 3(-3.51 \times 10^{-11})\,\Delta T^2 + \frac{1.16 \times 10^{-6} + 2(1.07 \times 10^{-9})\,\Delta T}{\lambda - 0.251}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$