Page:Philosophical Transactions - Volume 003.djvu/216

 see what is almost incredible. Besides which there are deliver'd in it the Practical Rules for working Spectacles for all sorts of sights, and especially Telescopical Glasses for observing the Planets and Fixed Stars by Sea and Land; and others, to magnifie the smallest of near Objects thousands of times.

There being, among the rest, described by the Author a particular way, call'd by him New and Universal, for making Convex-glasses upon a Plain for all practicable lengths of Diameters of Spheres, without other dishes or concave Moulds; we shall, for the satisfaction of some of the Curious, that are not Masters oi the Italian Tongue, and desire to know, what this way is, give it them in English, thus;

To give a Spherical Figure to a Plain by a Plain, (which at first sight may seem at Paradox) by moving one Plain upon another by a circular Motion, proceed to this manner: Let the piece to which the Glass to be wrought is fastned, be adjusted in the head of a Pole, which is to be of the length, the Semi-diameter of the Sphere of the Lens requireth; and, upon the Stool or Form, where you intend to work, let there be put a Plain of Iron or other Mettal, level to the Horizon; and perpendicularly over this Plain let the Pole be fasten'd to the Ceeling of the Room, if it be high enough; or to another steddy fastning, lower than the Ceeling, if need be, after this manner: About the head of the said Pole let there be fa{ten'd a Frame, made of two concentric rings or hoops, so that the one be mov'd within the other upon two Poles, and this other upon other Poles, movable between two small arms fixt to the Ceeling: which Frame you may imagine like that by which the Mariners Compass is kept Horizontal, or that, which they use in Italy for carrying Oyl-lamps by night Horizontally: Or the same may be done with a Ball moveable within two Circles fixed, and fasten'd on the top of the said Pole. All which will be better understood by the Figures, in the first of which, the Lens is T, cemented to the Piece E, fasten'd to the Pole S, which passes through the Center of the inner Circle B, moving upon the pivots I. H. (Fig. 2) in the outer Circle A; and this is fasten'd in a Frame upon the pivots L M, in the arms C D, fixt in a wall, or above in the Ceeling (as hath been said) according to Rh