SK55 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.15265\,\lambda^2}{\lambda^2 - 0.00582679} + \frac{0.432145\,\lambda^2}{\lambda^2 - 0.0190245} + \frac{1.03215\,\lambda^2}{\lambda^2 - 88.175}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.35 \times 10^{-6} + 2(6.03 \times 10^{-9})\,\Delta T + 3(4.4 \times 10^{-11})\,\Delta T^2 + \frac{3.3 \times 10^{-7} + 2(4.08 \times 10^{-10})\,\Delta T}{\lambda - 0.202}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$