N-SK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.28189\,\lambda^2}{\lambda^2 - 0.00727192} + \frac{0.257738\,\lambda^2}{\lambda^2 - 0.0242824} + \frac{0.968186\,\lambda^2}{\lambda^2 - 110.378}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.8 \times 10^{-6} + 2(1.41 \times 10^{-8})\,\Delta T + 3(2.28 \times 10^{-11})\,\Delta T^2 + \frac{6.44 \times 10^{-7} + 2(8.03 \times 10^{-11})\,\Delta T}{\lambda - 0.108}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$