lyformed^yet would only reflea thofe rays truly, which re- 
ipcd: iho axtr. ^Ihe errours of the (aid convex will be nitich 
augmented by the too great diftance, through which the rays , 
reflcded from it , rouft pafs before their arrival at the Eyel 
glafs. For which reafon i find it convenient to make the Tube 
ijo wider than is ncceflaryj that the Eye glafs be placed as 
near to the Oval planej as is poffiblej without obftruding any 
ufeful light in its pafl'age to the objed metaK 4. The errors of 
the objed-metal will be more augmented by reflexion from 
the convex than from rheplane^ becaufe of the inclination or 
deflexion of the convex on all fides from the points, on which 
every ray ought to be incident. 5* For thefe reafons there is re- 
quifite an extraordinary exadnefs in the figure of the little 
conv€X5 whereas 1 find by experience^ that it is much mote- 
difficult to communicate an exad figure to fuch fmall pieces of 
Metal^ than to thofe that are greater* 6 Becaufe the errors at 
the perimeter of the concave Objed- Metal , caufed by the 
Sphericalnefs ofits figure^are much augmentedby the convex, 
it will not with difl:indnefs bear fo large an aperture^as in the 
other conltruftion. By reafon that the little convex condu- 
ces very much to the magnifying virtiie of the inflrument, 
which the Oval plane doth notj it will magnify much more in 
proportion to theSphere^oa wliich the greatconeaveis ground, 
than in rh^ other defignj And fo magnifying Objeds much 
more than it ought to do in proportion to its aperture,it mufl: 
reprefent them very obicure and dark > and not only fo, but 
alfo coafufed by reafon of its being overcharged. Nor is 
there any convenient remedy for this. For, if the little con- 
vex be made of a larger Sphere , that will caufe a greater in- 
convenience by intercepting too many of ^he bcfl: rayes ; or,if 
the Charge of tlic Eye- glafs be made fo much fliallower as is 
necefrary,the angle of vifiou will thereby become fo little^tfaat 
k will be very difficult and troublefome to €nd an objed, and 
©f that objed, when found, there will be but a very fmall part 
leen at once. 
By this you may perceives that the three advantages^ which 
Monfieur Cajpgrmn propounds to himfelf, arc rather difad. 
wancagci* For, according to his delignj^the aperture of the 
iuftrumenc 
