( »95 )

7< M. Borelli hath found out a (ure and very easie Method to work.allfort of Areat Glasses. He hath already made one of them very good ofxoo Foot, wrought on both sides on the fame Rule. His Desire of advan-cing Astronomical Diseoveries hath induced him to make Presents of them toseveral Persons capable to make use of them. He hath iutrusted the Secretrto one of the Royal Acadcmy of Sciences.

Campani and Divini have Commonly seid their Glasses at a Pistol the Foot.Sometimes they have far exceeded that Price. One of Divinfs of 12 Footwas seid for 400 Livers; and another of Campanis , of 54 Foot, for 2000Livers. Notwithstanding which 5 . Borelli is willing to part with the best ofhis own Glasses, of jo, 6 o, or 6y Foot, for yoo ( French ) Crowns ; andthe (mali Glafles, from 6 to iz Foot, at a (French) Crown a Foot ; frorti12 to 18, at half a Pistol; and from 18 to 26, at a Pistol.

8. Though it be commonly believed, that Rsc< Cryflal is not fit for Op-tick Glasses, because there are many Veins in it ; yet Eustachio Divini madeone of it, which he faith proved an excellent one, though füll of Veins:but perhaps they were only superficial Strictrures and slight Scratches, notVeins.

9. Drops of fair Water bring let fall onapiece of plain Glast, form them-lelves mto Plano-convexes, having a Convexity proportionable to the heightsfrom which they descend; from a greater height a lest, from a lest, a greaterdegree of Convexity. I applied some of these as Reading Glafles for singleWords of ffnall Letters, as on the Globes and Maps, and found no otherInconveniency, than that the Fluidity of the Water obliges one to keep theGlast Horizontal, which I aster devifed a way to remedy. I took a seflicientquantity of Izing-Glaß, and dissolved it in Water over theFire, and whilftit was warm I dipt a Stick into the Solution, and let some Drops of it fall onthe Glast as before ; and in a quarter of an Hour they acquire a Consistency,that permits them to be held in any Position,, and tho’ they are not altogetheiso transparent, yet this is litrfe or no Impediment to their Use. The Dropsof this Solution are more exactly defincd 1 than those of common Water, ha-ving their edgesexactly Circular, and one may make them of a mueh longerFocus than tnose.

A thin flat Ring of Braß, not exceeding 4 tenths of an Inch Diameter inIts interior Circle, being cemented to a plain piece of Glast, and fi'lled widiWater, or the Solution now mentioned, thervby pressmg the Finger into it,till wliat is fuperfluous be taken off, there will be formed a Plano-concave,which may serve as an Eye Glast to a Per spe clive, or to any other Opticaluse Concave Glafles are applicable.

I have tried what would be the Succest of combining Portions of Waterby tbe help of Braß Rings, and plain pieccs of Glast* to give them theirtrue Figure and requisite Apertures, and insertcd them at the ends of Tubesof several Lengths; and find* that tho v these Natural Lentes may serve asEye Glasses, yet when used as Objedl ones, ekher to Telefcopes or doubleMicrofcopes, their Effects will not compenstte the Trouble there is in usingthem.

G c r. XVH. i.When

By .Vf.BßreUln. 128. f.

n. 140. f.1005.

Opuct^Lcm'sof Rod^ Cryjlal ;fcyEust.Divini.n. 20. z62-

Of Water ; byMr. StephenGray. n. 22S.589 -