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Appendix II.

a nd the Theorem for a double and equally

c onvex Lens will be —^-= f which

2. ad—r

^vr parallel Rays, becomes ~~f- } whence

2 a

r = 2 af\ oxs\ r :: I : 2 a.

Therefore to represent this Matter inl ts proper Light, we will state the Propor-tions of Page 378 from this correct Theo-re m, and they will stand thus, d : f ::Zad —r: r; andd +f :f :: 2 ad :r\ that is,^ Qj QN :: 2 ixAN:NL:: AB:^ O : Hence N O : A B :: r : 2 a d ::f: d.0 ^, the Distance of the Centers N O has the•f'^ne Ratio to A B, the Length of the ObjeSl ,as the focal Distance of parallel Rays has to*he Distance A N, or as the Radius has to thefold Distance multiplied by 2 a. Whence ’tisev ident we cannot use the Radius without con-

sidering the Refraction of the Glass ; but thef°cal Distance of parallel Rays serves equallyf °r all Sorts of Lenses, is most easily found,^d therefore fittest for the Analogy, as be-f °re determined, in measuring Angles and^stances.

With regard to the Planets, as fupiterthe largest of all, and subtends an Anglet° the Eye of 3 1 12", the Diameter of hisWge in the Focus of a 50 Foot Glass will

be