3n1 is the refractive index of the third medium with respect to the first medium.
∴ 3n1 = 2n1 × 3n2.
[Suppose medium 1 = air, medium 2 ≡ ice and medium 3 ≡ diamond. Then, 2n1 ÷ 1.31, 3n2 = 1.847
∴ 3n1 = 2n1 × 3n2 = 1.31 × 1.847 = 2.42 which is the refractive index of diamond with respect to air.]