η = B′ b C the second angle of refraction
ε = B b B′ the first angle of refraction
i = B′FA the first angle of incidence
i′ = BFA
e = b′ B b
AB = r
AB′ = r′
B b = t″ the thickness of the lens at the edge
AF = φ the focal distance.
Then tan i′ = rφ; sin e = sin i′μ whence b b′ = t″ tan e becomes known.