Concerning his Advertisement to such, as publish _Theories_, I find not, that he hath made use of it in his own case. For, in his _Theory_ about _Apertures_ he seems to be very positive, not at all doubting to rely upon it, _vid._ that the _Apertures_ must be _thus_ and _thus_ in _great_ Glasses, because he had found them _so_ or _so_ in some _small_ ones. For his Proposal of amendments of some inconveniencies in this way, I return him my thanks; but as to his first I believe, that the matter may be conteined as wel in the _Concave_ Tool, as on the _convex_ Glass. And as to that of 2 _Poppet-heads_ I do not well understand it, if differing from mine; and the keeping of the Tool upon the Glass with a spring or weight, must quickly spoyl the whole; since, if either of the _Mandrils_ will easily yield backwards, the _regularity_ of _all_ will be spoiled: and as to the wrigling and playing of the _Mandril_, I do not at all apprehend it. {69} His _Theory_ of _Apertures_, though he seems to think it very authentick, yet to me it seems not so cleer. For, the same Glass will endure greater or lesser _Apertures_, according to the lesser or greater Light of the _Object_: If it be for the looking on the _Sun_ or _Venus_, or for seeing the _Diameters_ of the _Fix'd Stars_, then smaller _Apertures_ do better; if for the _Moon_ in the _daylight_, or on _Saturn_, or _Jupiter_, or _Mars_, then the largest. Thus I have often made use of a 12 foot-Glass to look on _Saturn_ with an _Aperture_ of almost 3 inches, and with a single Eye-glass of 2 inches _double convex_: but, when with the same Glass I looked on the _Sun_ or _Venus_, I used both a smaller _Aperture_, and shallower _Charge_. And though M. _Auzout_ seems to find fault with the _English_ Glass of 36 foot, that had an _Aperture_ of but 23/4 inches _French_; as also, with a 60 foot _Tube_, used but with an _Aperture_ of 3 inches; yet I do not find, that he hath seen Glasses of that length, that would bear greater _Apertures_, and 'tis not impossible, but his _Theory_ of _Apertures_ may fail in longer Glasses. * * * * * _Of a means to illuminate an Object in what proportion one pleaseth; and of the Distances requisite to burn Bodies by the _Sun_._