N-SK4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.32994\,\lambda^2}{\lambda^2 - 0.00716874} + \frac{0.228543\,\lambda^2}{\lambda^2 - 0.0246456} + \frac{0.988465\,\lambda^2}{\lambda^2 - 100.886}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.96 \times 10^{-7} + 2(1.3 \times 10^{-8})\,\Delta T + 3(-1.31 \times 10^{-11})\,\Delta T^2 + \frac{4.36 \times 10^{-7} + 2(6.01 \times 10^{-10})\,\Delta T}{\lambda - 0.179}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$