( '93 )

XV. Prepare two Glafles, thc one exactly Hat on both fides, the other flat Toon thc one fide, and Convex on the other, of what Spherc you pleafe. Letthe flat Glafs be a little broader than the other. Then let there be made a cotuA </'Gell or Ring of Brali, very exactly turned, into which thefe two Glafles may p-

bs so fastned with Cernent, that the Plain Surfaces ot them may lie exadbly'HooU . uParallel, and that thc Convex fide of the Plano-convex Glafs may lie inward ; but p. 66 . -

so, as not to touch the Flatos the other Glafs. Thefe being Cemented into the P' 2o: '

Ring very clofely about thc Edges, by a finali Hole in the fide of the Braf -Ring or Cell, fili the interpofed Space between thefe two with Water, Oylof Turpentine, Spirit of Wine, Saline Liquors, &c ; then stop the Holewith a Skrew : And according to the diftering Refraction of the InterpofedLiquors, so stiall the Focus of this Compound Glafs be longcr or fhorter.

But this I would have only lookt upon, as one Instance of many (fort’nere may be others) of thc Poilibility of making a Glafs, ground in afmallcr Sphere, to constitute a Telefcopeof a much greater Length : Though(not to raife too great Expectation) I must add, That, of Spherical ObiectGlaflcs thole are the best which are made of the greatest Spherc, and wholcSubstance hath the greatest Refraction.

. XVI 1. S. Campani pretends to have found a Way to Work great Optidp ^hfsOpn™Glaflcs with a Turn-tool, without any Mould: And that hc ufeth three Eye GUfftr, fcvCam-Glafles for his great Telefcopes, without finding any Rainbow Colours. P ani ’ D( V1 '

The Great Duke of Tufcany , and Prince Leopold his Brother, upon Tryal made ^ lof the Glafles of Campani and Divini , have found that thole of Campani ' ' f ’ 13 ‘excel the other ; and with them they luve been easily able to distinguifhPcople at four Leagues dillance.

But Eustachio Divini pretends, that in all the Tryals made with them, n. 12. p. 209-his great Glafles have performed bett er than thole of Campani and thatCampani was not willing to do what was neceflary for well comparing theone with the other, vi\. to put Equal Eye Glafles in them, or to cxchangethe fame Glafles.

1. ’Tis now above 10 Years since I invented a peculiar way of Grind-ing Optick Glaflcs, and reduccd it also into Practicc ; by which 'tis easie, with-out any considerable danger of failing, to make and polisti Optick-glafles ofany Conicis Scii ion, and that (which is most notable) in any Disti ot any Sc-cRon of a Sphere. I have already made fcveral Glafles by it, which manyLearned Men have feen and trycd.

M. Huygcns also intends very fhortly to try fomething in that kind.

Z Ai. du Sons doth at prefent cmploy himfclf in London, to bring Telef-copes to perfection, by Grind ing Glafles ot a Parabolical Figure. i have feentwo Eye Glafles of that fhape, about one inch and a half deep, and one inchand a quarter broad, wrought by this eminent Artist with a rare Steel Instru-ment ot his own Contrivance and VVorkmanfhip, and by hlmlelf also polisti'dto Admiration. And certainly it will be wondred at by thole, who ihall (eethefe Glafles, how they could be truly wrought to lucii a Figure, with

By As. Hevcli-us. n. a.

p. 98.

By As. Huy-gens. ii>.

By As. <fu Sonsib. p. 99 . 11. 7.P. i I <7.

Vok 1

Cc

such