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215)

[[left margin]] On the improvement of Telescopes. See p. 70 to 80. [[/left margin]]

At p. 70 is an account of Dollond's improvement of Refracting Telescopes. In the Philosophical Transactions for 1765 is a farther accout of improvements therein by M [[superscript]] r [[/superscript]]. Dollond, the Son, and describes one whose compound object glass consists of two convex lenses of crown glass, and a concave lense of white flint glass placed between them, having their common [[underlined]] focus [[/underlined]] 3 1/2 feet distance, bearing an aperture of 3 1/2 inches, and magnifying with distinctness and clearness 150 times; i.e. as much nearly as a good telescope of the common sort, of 40 feet.  As M [[superscript]] r [[/superscript]]. Dollond has not given the [[underlined]] radii [[/underlined]] of the curvature of these glass, tis [[underlined]] supposed [[/underlined]] that he hit upon this construction by mere practical trials, and may not therefore given them the proper dimensions to produce the greatest & best effect. - It is shewn in the Hist. of the Royal Acad. of Sciences at Paris, for the year 1764 by M. D'Alembert, from calculations, that that an equal effect may be produced by an object-glass compounded of a [[underlined]] meniscus [[/underlined]] of [[strikethrough]] flint glass [[/strikethrough]] crown glass, a convex lens of the same matter, with a [[underlined]] meniscus [[/underlined]] of flint glass interposed. Also in one of M. D'Alembert's proposed compound object-glasses, the aberration arising from the Spherical figure is not greater than in a reflecting telescope of the same magnifying power; and that the aberration of refrangibility is reduced to an hundreth part of that produced by a single convex lens of crown glass of the same focal length; and that even this small quantity may be made to disappear by certain methods, which he proposes to communicate in a subsequent Memoir.

Transcription Notes:
minor edits. -megshu

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