FCD1B by Hoya

$$n(\lambda) = \sqrt{2.21891 - 0.00617071\,\lambda^2 + 0.00808876\,\lambda^{-2} + 0.000133578\,\lambda^{-4} + 1.94762 \times 10^{-7}\,\lambda^{-6} - 2.25778 \times 10^{-7}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.75 \times 10^{-5} + 2(-5.28 \times 10^{-9})\,\Delta T + 3(2.39 \times 10^{-11})\,\Delta T^2 + \frac{3.52 \times 10^{-7} + 2(1.97 \times 10^{-10})\,\Delta T}{\lambda - 0.186}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$