And this may suffice for an Introduction to Readers of quick Wit and good Understanding not yet versed in Opticks: Although those who are already acquainted with this Science, and have handled Glasses, will more readily apprehend what followeth. FOOTNOTES: [A] In our Author's _Lectiones Opticæ_, Part I. Sect. IV. Prop 29, 30, there is an elegant Method of determining these _Foci_; not only in spherical Surfaces, but likewise in any other curved Figure whatever: And in Prop. 32, 33, the same thing...