(631)
An Account
The ingenious and industrious Francis Smethwick Esquire, Fellow of the Royal Society, having for divers years painfully search'd after the way of grinding Glasses not-Spherical, affirms, that at length he hath now found it; for the proof of which, he lately (viz. February 27. 16678.) produced before the said Society certain Specimina of that Invention, which were a Telescope, a Reading and two Burning-Glasses.
The Telescope was about four foot long, furnish'd with four Glasses, whereof the three Ocular ones, Plano-convex, were of this newly-invented not-Spherical Figure, and the fourth a Spherical Object-glass. This being compared with a common, yet very good Telescope, longer than it by about four inches, and turned to several Objects, was found by those of the said Society that look'd through them both, to exceed the other in goodness, by taking in at greater Angle, and representing the Objects more exactly in their respective proportions, and enduring greater Aperture, free from Colours.
The Reading Glass of the same Figure being compared with a common Spherical-Glass, did far excell it, by magnifying the Letters, to which it was applied, up to the very edges, and by shewing them distinctly from one brim through the Center to the other, which the Spherical Glass came far short of. And this effect the new-figur'd Glass perform'd only on one of its sides, and not on the other, as being of a different figure from Spherical-Glasses, which perform their effect near-equally on both sides.
Lastly, The two Burning-Concaves of this new-invented figures were, the one of six inches Diameter, its focus three inches distant from the Center thereof; the other of the same Diameter, but less concave, and its focus 10 inches distant. These, when approach'd to a large Candle lighted, did somewhat warm the faces of those, that were 4 or 5 foot distant at least, and when
