( 4oc8 ) 
So far the Inventors Letters touching this Infirummt : of which 
having communicated the defcription to Monfieur Chrijiian 
Hugens de Z^lichm^ we received from him an Anfwer to this 
efFedi, in his Lcttcr of Febr. ig* 1672, ft.n* 
I fee by the Defcription, you have feot me of Mr. Newtons 
admirable Telefcon% that he hath well confidered the advan- 
tage, which a Concave fpeculum hath above Convex ghjfes'm 
colleaing the Darallel rays, which certainly according to the 
calculation, 1 have made thereof, is very great. Hence it i 
is, that he can give a far greater aperture to that fpeculum^ 
than to an Objed-glan of the (ame diftance of xht focus, and 
confequently that he can much more magnifie objefts this 
way, than by an ordinary Tclefcope. Befides , by it he a- 
voids an inconvenience^ which is infeparable from convex 
Objea-Glaffes, 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 refledion of the metalliu 
Jpemlum there are not fo many rays loft^ as in GlaflTes, which 
refled a confiderable quantity by e^ch of their furfaces, and 
bclides intercept many of them by the obkurity of their 
matter* 
Mean timCj the main bufinefs will be , to find a matter for 
this [pe$uln?n that will bear fo good and even a polifli as Glaf- 
fes , and a way of giving this polifh without vitiating the 
fpherical figure. Hitherto I have found no ty/^ra/^, that had 
near fo good a polifli as Glafs ; and if M. Newton hath not 
already found a way to make it better, than ordinarily f ap- 
prehend, his Telefcopes will not fo well diftinguifh objects, 
a$ ihofe with Glaflcs. But 'tis worth while to fearch for a 
remedy to this inconvenience , and I defpair not of finding 
OQC, I believe, that M^lSlewton hath not been without con- 
fidering the advantage 5 which a Parabolical fpeculum would 
have above a vS/^i'mV^/ one in this conftrudiion ^ but that he 
defpairs, as well as I do j of working other furfaces that! 
fpherical ones with due exaftnefs ; though elfe it be more 
eafie to make a Parabolical than Elliptical or Hyperbolical ont$, 
byreafonof acertsin propriety of the P<?r«^(?//V^r«?m4 which 
... is, 
