SF59 by Schott

$$n(\lambda) = \sqrt{1 + \frac{2.05776\,\lambda^2}{\lambda^2 - 0.01678} + \frac{0.528645\,\lambda^2}{\lambda^2 - 0.0655336} + \frac{1.00044\,\lambda^2}{\lambda^2 - 110.003}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.67 \times 10^{-6} + 2(1.59 \times 10^{-8})\,\Delta T + 3(-7.12 \times 10^{-11})\,\Delta T^2 + \frac{2.51 \times 10^{-6} + 2(2.49 \times 10^{-9})\,\Delta T}{\lambda - 0.289}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$