He adds, that in that metall, which he sent to London, there was no Arsenick, but a small proportion
of Silver; as he remembers, one shilling in three ounces of metall. But he thought withall, that the
Silver did as much harm in making the metall soft, and so less fit to be polish't, as good in rendring it
white and luminous.
At another time he mixed Arsenick one ounce, Copper six ounces, and Tin two ounces: And this an
Acquaintance of his hath, as he intimates, polish't better, than he did the other.
As to the objection, that with this kind of Perspectives, objects are difficultly found, he answers in
another letter of his to the Publisher, of Jan. 6. 1671/72. that that is the inconvenience of all Tubes
that magnifie much; and that after a little use the inconvenience will grow less, seeing that himself
could readily enough find any day-Objects, by knowing which way they were posited from the other
objects that he accidentally saw in it; but in the night to find Stars, he acknowledges it to be more
troublesome; which yet may, in his opinion, be easily remedied by two sights affixed to the Iron rod,
by which the Tube is susteined; or by an ordinary perspective glass fastn'd to the same frame with
the Tube, and directed towards the same object, as Des-Cartes in his Dioptricks hath described for
remedying the same inconvenience of his best Telescopes.
<(4008)>
So far the Inventors Letters touching this Instrument: of which having communicated the description
to Monsieur Christian Hugens de Zulichem, we received from him an Answer to this effect, in his
Letter of Febr. 13. 1672. st.n.
I see by the Description, you have sent me of Mr. Newtons admirable Telescope, that he hath well
considered the advantage, which a Concave speculum hath above Convex glasses in collecting the
parallel rays, which 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, than to an Object-glass of the same
distance of the focus, and consequently that he can much more magnifie objects this way, than by
an ordinary Telescope. Besides, by it he avoids an inconvenience, which is inseparable from
convex Object-Glasses, which is the Obliquity of both their surfaces, which vitiateth 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 reflection of the metallin speculum there are not so many rays lost, as in
Glasses, which reflect a considerable quantity of each of their surfaces, and besides intercept many
of them by the obscurity of their matter.
Mean time, the main business will be, to find a matter for this speculum that will bear so good and
even a polish as Glasses, and a way of giving this polish without vitiating the spherical figure.
Hitherto I have found no Specula, that had near so good a polish as Glass; and if M. Newton hath
not already found a way to make it better, than ordinarily I apprehend, his Telescopes will not so
well distinguish objects, as those with Glasses. But 'tis worth while to search for a remedy to this
inconvenience, and I despair not of finding one. I believe, that M. Newton hath not been without
considering the advantage, which a Parabolical speculum would have above a Spherical one in this
construction; but that he despairs, as well as do I, of working other surfaces than spherical ones
with due exactness; though else it be more easie to make a Parabolical than Elliptical or
Hyperbolical ones by reason of a certain property of the Parabolick Conoid, which <(4009)> is, that
all the Sections parallel to the Axis make the same Parabola.
Thus far M. Hugenius his judicious Letter; to the latter part of which, concerning the grinding
Parabolical Conoids, Mr. Newton saith, in his Letter to the Publisher of Feb. 20. 71. that though he
with him despairs of performing that work by Geometrical rules, yet he doubts not but that the thing
may in some measure be accomplished by Mechanical devises.
To all which I cannot but subjoyn an Extract of a Letter, received very lately, (March 19th) from the
Inventor of this new Telescope, from Cambridge, viz.
IN my last Letter I gave you occasion to suspect, that the Instrument which I sent you, is in some
repect or other indisposed, or that the metals are tarnished. And by your Letter of March 16. I am
fully confimed in that opinion. For, whilest I had it, it represented the Moon in some parts of it as
distinctly, as other Telescopes usually do which magnifie as much as that. Yet I very well know, that