PK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.20922\,\lambda^2}{\lambda^2 - 0.00733975} + \frac{0.0668873\,\lambda^2}{\lambda^2 - 0.0286496} + \frac{1.01818\,\lambda^2}{\lambda^2 - 101.853}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.32 \times 10^{-6} + 2(1.09 \times 10^{-8})\,\Delta T + 3(-3.17 \times 10^{-11})\,\Delta T^2 + \frac{3.47 \times 10^{-7} + 2(2.75 \times 10^{-10})\,\Delta T}{\lambda - 0.225}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$