PK51A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.15611\,\lambda^2}{\lambda^2 - 0.00585597} + \frac{0.153229\,\lambda^2}{\lambda^2 - 0.0194072} + \frac{0.785619\,\lambda^2}{\lambda^2 - 140.537}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.98 \times 10^{-5} + 2(-6.06 \times 10^{-9})\,\Delta T + 3(1.6 \times 10^{-11})\,\Delta T^2 + \frac{4.16 \times 10^{-7} + 2(5.01 \times 10^{-10})\,\Delta T}{\lambda - 0.134}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$