Christiaan Huygens' comments on Newton's telescope

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.