SSK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.41096\,\lambda^2}{\lambda^2 - 0.00805433} + \frac{0.172426\,\lambda^2}{\lambda^2 - 0.0324361} + \frac{0.987462\,\lambda^2}{\lambda^2 - 109.9}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.11 \times 10^{-6} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-1.46 \times 10^{-11})\,\Delta T^2 + \frac{4.7 \times 10^{-7} + 2(5.41 \times 10^{-10})\,\Delta T}{\lambda - 0.239}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$