( '99 )

z. I lee by the Deseription you have scnt me of Mr. Norton s admirable b ^

Teleseope, that he hath weil considered the Advantagc, which a Concave zälichem.Speculum hath above Convcx Glasscs in collecting the Parallel Rays, which /W. p. 4008.certainly according to the Calculation I have made thereof is very great.

Hence it is, That he can give a far greater Aperture to that Speculum, thanto an Object: Gials of the lanie distance of the Focus, and conlequently thathe can much more Magnifie Objects this way, than by an ordinary Telescope.

Besides, by it he avoids an Inconvenience, which is in sepa rabie frora Con-vex Object Glasscs, which is the Obliquity of both their Surfaces, whichvitiateth the Refraction of the Rays that paß towards the sides of the Glaß»and does more hurt than Men are aware of. Again, by the meer Reflexionof the Metalline Speculum there are not lo many Rays lost, as in Glasses,which Reflect a considerable quantity by cach of their Surfaces, and besidesintercept many of them by the Obseurity of their matter.

Mean time, the main businels will be, to find a matter for this Speculumthat will bear so good and even a Polilh as Glafies, and a way of givingthis Polilh without vitiating the Spherical Figure. Hitherto I have found noSpecula that had near so good a Polilh as Glaß : And if Mr. Newton hath notalready found a way to make it better than ordinarily, I apprehend his Te-lefcope will not so well distinguilh Objects as thole with Glasses. But ’tisworth while to fearch for a Remcdy to this Inconviency, and 1 delpair not offinding one. I believe that Mr. Newton hath not been without consideringthe Advantage, which a Parabolical Speculum would have above a Sphericalone in this Construction ; but that he delpairs, as well as I do, of workingother Surfaces than Spherical ones with due exactnesi ; though eise it bemore easie to make a Parabolical than Elliptical or Hyperbolical ones, by rea-lön of a certain Propriety of the Parabolick Conoid, which is, that all theStctions Parallel to the Axis make the tarne Parabola.

But though Mr. Newton (with M. Hugens) delpairs of performing thatwork by Geometrical Rules, yet he doubts not but that the thing may insome measore be accomplilhed by Mechanical Devices.

4. In my last Letter, I gave you occasion to solpect, that the Instrument f»rtb<r ot.-

which I scnt you is in some relpect or other indifpofed, or that the Metalsare Tarnilhed : And by yours I am fully confirmed in that Opinion. For, Mr. Newton,whilst I had it, it reprelentcd the Moon in some Parts of it as distinctly asother Teleseopes usoally do which Magnifie as much as that. Yet 1 verywell know, That that Instrument hath its Imperfections both in the Compo-fition of the Metal, and in its being badly Gast, as you may pereeive by aScabrous place near the middle of the Metal of it on the Polilhed side, andalso in the figure of that Metal near that Scabrous place : And in all thoserelpects that Instrument is capable of further Improvement.

You seem to intimate, That the Proportion of 38 to 1, holdsonly for itsMagnifying Objects at Imalldistarrces. But if for such Distances, soppose jooFeet, it Magnifie at that rate, by the Rules of Opticks it must for the grat-est distance imaginableMagnifie more than 37J to 1, which is so inconsidera'ble a diminilhing, that it may be evea thea as 58 to. .1,

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