(ty 
Will be worn orgrownd away/o as that a Ring of an inch broad 
may be made to touch the Spherical Surface of the Glafs ; nay, 
if it be neceflary^wi'thout much trouble, efpecially in the grind- 
ing of longer Glafses) the whole Concave Surface of the Tool' 
may be made to touch a Glafs.;Befides,that as to the keeping a 
quantity of the fame fand and Powders of feveral finefses , ac- 
cording as the glafs wears, the fame is poffible to be don. as with 
the fame Sand wrought finer by working in the Ordinary way. 
The giving the Inclination to the Mandrils , is not at all diffi- 
cult* though perhaps to determine the length exadtiy which 
the Glafs fo made fliall draw, is not fo eafie .• But his no matter, 
what length the Glafs be of, foit be made good, whether 60 
or 80 foot, or the like. Nor is it fo very difficult, to lay them 
both in the fame Plain. And to keep them fleddy , when once 
fix'd, is moft eafie. 
As to the Calculation of the propriety of a Glafs of a thou- 
fand foot, perhaps for that particular Length, I had not, nor 
have as yet calculated, that the Convexity of one of eighteen 
inches broad, will not be above a feventh part of a Line. But 
it does not thence follow , that I had not confidered the diffi- 
culties, that would be in making of it. For,! muff tell him, that I 
can make a Plano-convex, Glafi, though its convexity be of a final- 
er fphere than is ufual for fuch a length, to be an ObjeCPGlaf of a- 
bout i j o foot in Length, nay of 300 foot, and either longer or 
fiiorter, without at all altering the convexity. So that, if he' will! 
by any Contrivance hehath,giveme ^Plano-convex Glafs of $ey 
or 40 foot Diameter ^ i thou t Veins, and truly wrought of that Ft - 
£ure : J will prefently make a 'Ttdefcope with ir,that with a fingle Ey- 
glafs fliall draw a thoufand foot : Which Invention , I fhall Ihortly 
difeover, there being, I think, nothing more eafie and certain. 
And if a Plano-convex Glafs can be made of any Sphere between 
twenty and fourty foot radius^ fo as that both, tlie Convex and; 
Plain fide of the Glafs be exactly poiifh’d of a true Figure^l wil! 
portly {hew, how therewith may be mad.® a T elefcope of any 
Length, fuppofing the Glafs free from all kind of Veins, or ine- 
quality of IfefraHion. 
As for the Aiding of the Glafs upon the Cement , I fee no rea- 
son at all for it, at leaft in the Cement^ I make ufe of, having ne- 
ver obferved any fuch accident in hard Cement . Anil 
