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_ and his Conclusion thereupon (_videl. That we are not to expect Glasses of above 300 or 400 foot long at most, and that neither _Matter_ nor _Art_ will go so far_) is, may be judged from what I have newly told you of making any _Object-Glass_ of any Length. And for his good wishes, that those, who promise to make him see _Plants_ or _Animals_ in the _Moon_ (of which I know not any, that has done so, though perhaps there may be some, notwithstanding his Objections, that do not yet think it impossible to be done) had considered, what a Man is able to see with his _bare_ Eye at 60 Leagues distance: I cannot but return him my wishes, that he would consider the difference between seeing a thing through the _Gross_ and _Vaporous_ Air neer the Earth, and through the Air over our heads: Which, if he observe the Moon in the _Horizon_, and neer the _Zenith_ with a _Telescope_, he will experimentally find; and, having done so, he will perhaps not be so dissident in this matter.