( 4008 ) 
So far t%e Inventors Letters touching this Injlrummt : of which 
having communicated the defcriptioo to MonCicuv Chr?Jiian 
Hugens de Z^lichem^ we received from him an Anfwertothis 
effed:, in his Letter oif Febr. 13* 1672. ft.n* 
I fee by the Dcfcription, you have fet)t me of Mr. Neivtons 
admirable Telefcope, that he hath well confidered the advan- 
tage, which a (foncave fpeculum hath above Convex gaffes in 
collediog the parallel rays, which certainly according to the 
calculation, I have made thereof, is very greats Hence it 
is, that he can give a far greater aperture to that fpeculum^ 
than toanObjeiSt-glafsof the fame diftanceof the focus, and 
confequently that he can much more magnifie objefts this 
way, than by an ordinary Telefcope. Befides , by it he a* 
voids an inconvenience^ which i$ infeparable from convex 
Objeft-GlafTes, which is the Obliquity of both their furfaces, 
which vitiateth the refraftion of the rays that pafs towards 
the fides of the glafs , and does more hurt than men 
are aware of. Again, by the meer reflection of the metallin 
fpeculum there are not fo many rays lofl, as in GlafTes, which 
refleft a confiderable quantity by each of their furfaces, and 
befides intercept many of them by the obfcurity of their 
matter^ 
McantimCj the main bufinefs will be, to find a matter for 
this fpeeuhmih2it. Vi\\\ bear fogood and even a poltflias Glaf- 
fes, and a way of giving this polifh without vitiating the 
fpherical figure. Hitherto I have found no Specula^ that had 
near fo good a polifh as Glafs ; and if M. Newton hath not 
already tound a way to make it better, than ordinarily f ap- 
prehend, his Telefcopes will not fo well diftioguifh objedls, 
as thofe with Glafics. But 'tis worth while to fearch for a 
remedy to this inconvenience , and I defpair not of finding 
one. I believe, that hUNewton hath not been without con- 
fidering the advantage , v>^hfch a Parabolwal fpeculum would 
have above a Spherical one in this conflru<flion 5 but that he 
dcfpairs, as well as I do, of working other furfaces thaa 
fpherical ones with due exaftnefs ; though elfe it be more 
cafie to make a Parabolical than Elliptical or Hyperbolical ones, 
by reafon of a certain propriety of the Farabolic\ f<?«^^*4 which 
