Bφ - Bβ = _mdr_/(_m - nd - nr_) - _t_,
putting _p_ for _n_/(_m - n_), after due Reduction this following
Equation will arise,
(_mpdrρ - ndρt + nprρt_)/(_mdr + mdρ - mprρ - m - ndt + nrt_)
= _f_.
Which Theorem, however it may seem operose, is not so, considering the
great Number of _Data_ that enter the Question; and that one half of the
Terms arise from our taking in the thickness of the _Lens_, which in
most Cases can produce no great Effect; however it was necessary to
consider it, to make our Rule perfect. If therefore the _Lens_ consist
of _Glass_, whose Refraction is as 3 to 2 'twill be
(_6drρ - 2dρt + 4rρt_)/(_3dr + 3dρ - 6rρ - dt + 2rt_) = _f_.