Fig. 55.
Referring to [fig. 55], we have, for obtaining the radius of the central disc, the following formulæ, in which
r = AB, half the aperture of the lens
r′ = AB′
φ = AF, the focal distance
t′ = A a, the thickness of the lens at the vertex
t″ = B b, the thickness of the joint
μ = the index of refraction
ρ = the radius of curvature.