K4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.18909\,\lambda^2}{\lambda^2 - 0.00789397} + \frac{0.0841359\,\lambda^2}{\lambda^2 - 0.0339219} + \frac{0.93022\,\lambda^2}{\lambda^2 - 107.049}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.85 \times 10^{-6} + 2(2.08 \times 10^{-8})\,\Delta T + 3(7.15 \times 10^{-11})\,\Delta T^2 + \frac{5.66 \times 10^{-7} + 2(5.14 \times 10^{-11})\,\Delta T}{\lambda - 0.186}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$