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Appendix II.

O and N are properly disposed, as beforementioned.

From what has been said, the generalRationale of this Micrometer will, I pre-sume, evidently appear; but one Thinmust not pass unregarded in an Affair cluch Moment and Consequence as thesuring these small Angles in the ScienceAstronomy, which has been customary tc>suppose, that so far the Focus of a T eIlS ’or the focal Distance of Rays parallel toAxis, is equal to the Radius in a doubleand equally convex Lens. But this is t°°great an Error not to be noticed here; s° rin different Sorts of Glass there is found 3different refractive Power, and the F°cU sof parallel Rays is at a different Distan c ^in each; but this Distance in no Sort 0Glass is equal to the Radius, but falls fb° rtof it more or less. Now the foregoingDemonstration regards the Radius, and n° £the focal Distance of parallel Rays.this Affair will be best illustrated byample as follows :

Let the Sine of Incidence be to the Sin eof Refraction out of Air into Glass a s

to n ; and let — n . — a, then will an

press the refractive Power of the Glass >r and