According to A. L. Cauchy’s formula, which represents the facts very well throughout most of the visible spectrum,
μ = A + Bλ−2
(10),
so that
| cot2 α′ = | 2B | = | 2(μ − A) |
| λ2μ | μ |
(11).
If we take, as for Chance’s “extra-dense flint,” B = .984 × 10-10, and as for the soda lines, μ = 1.65, λ = 5.89 × 10-6, we get
α′ = 79°30′.
At this angle of refraction, and with this kind of glass, the retardation of phase is accordingly nearly independent of wave-length, and therefore the bands formed, as the thickness varies, are approximately achromatic. Perfect achromatism would be possible only under a law of dispersion
μ2 = A′ − B′λ2.