K3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.08999\,\lambda^2}{\lambda^2 - 0.00670234} + \frac{0.18185\,\lambda^2}{\lambda^2 - 0.0254233} + \frac{0.936479\,\lambda^2}{\lambda^2 - 113.486}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-7.2 \times 10^{-7} + 2(9.17 \times 10^{-9})\,\Delta T + 3(-3.02 \times 10^{-11})\,\Delta T^2 + \frac{5.26 \times 10^{-7} + 2(6.52 \times 10^{-10})\,\Delta T}{\lambda - 0.19}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$