ccording as the glass wears, the same is possible to be
don, as with the same Sand wrought finer by working in the Ordinary way.

The giving the _Inclination_ to the _Mandrils_, is not at all difficult;
though perhaps to determine the length exactly which the Glass so made
shall draw, is not so easie: But 'tis no matter, what length the Glass be
off, so it be made good, whether 60 or 80 foot, or the like. Nor is it so
very difficult, to lay them both in the same _Plain_. And to keep them
_steddy_, when once fix'd, is most easie.

As to the Calculation of the propriety of a Glass of a thousand 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
seventh part of a Line. But it does not thence follow, that I had not
considered the difficulties, that would be in making of it. For, I must
tell him, that I can make a _Plano convex_ Glass though its convexity be a
smaler sphere than is usual for such a length to be an _Object-Glass_ of
about 150 foot in Length, nay of 300 foot, and either longer or shorter,
_without_ at all _altering the convexity_. So that, if he will by any
Contrivance he hath, give me a _Plano-convex_ Glass of 20, or 40 foot
_Diameter_, without _Veins_, and truly wrought of that _Figure_, I will
presently make a _Telescope_ with it, that with a single Ey-glass shall
draw a thousand foot: Which _Invention_, I shall shortly discover, there
being, I think, nothing more easie and certain. And if a _Plano-convex_
Glass can be made of any _Sphere_ between twenty and fourty foot _radius_,
so as that both the _Convex_ and _Plain_ side of the Glass be exactly
polish'd of a true _Figure_, I will shortly shew, how therewith may be made
a _Telescope_ of any Length, supposing the Glass free from all kind of
_Veins_, or inequality of _Refraction_.

As for the sliding of the Glass upon the _Cement_, I see no reason at all
for it, at least in the _Cement_, I make use of, having never observed any
such accident in hard _Cement_. {68}

And for the Bearing of the _Ring_ against one side of the Glass only at a
time, I cannot see, why _that_ should produce any inequality, since all the
sides of the Glass have successively the same pressure.

His ratiocination concerning a Glass of 300 foot, is much the same with the
former, about the difficulty of working a true surface of a convenient
figure; which how considerable both _that