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150 on one ide and flat on the other, in cae the flat Side be turn'd towards the Object, the Error which aries from the Contruction and Poition of the Glas, is above five thouand Times les than the Error which aries from the Refrangibility: And therefore, that the Shape or Figure of the Glaes is not the Caue why Telecopes cannot be carried to a greater Perfection, but aries wholly from the Nature of Light. this Reaon he invented a Telecope, which dicovers Objects by Reflection and not by Refraction. Telecopes of this new kind are very hard to make, and their Ue is not eay. But according to the Englih, a reflective Telecope of but five Feet, has the ame Effect as another of an hundred Feet in Length. LETTER