as the Turt2\% cortipofcd. For, when the Glaffes do wear , e- 
fpccially when they are very convex^ it cannot be otherwife, but 
the Mandril will play and wrigle, before the Scrue be made 
firm. 
But he doubts, whether all can be remedied^ which he ieavs^ 
to the induftry of Mr. confidering what he faith in the 
Preface oi his Micrography, touching a Method, he knows, of 
finding out as much in Muhanicks^ as can be found in Geometry 
by Algibra^ 
Befides this^ he taketh notice, that moft of thofe that mcdie 
with Optie\GlajJes^g\vtihQmviot2L%m\xch nor charge 
them (o deep as they ought. And he inftanccs in the Telefcope^ 
which Wis Majefly q£ Great Britain ^vciQnied the Du]^ of Or- 
leans with, videLxhdit it did be^r but 2 inches, and 9 lines French, 
for its greatefl: Aperture, though there be 5 or leffer Apertures^ 
of which it feems faith hi ) the Artificer would have thofe ^ 
that ufe it, ferve themfelves more ordinarily, than of the great- 
eftj which conveys but almoft half as many Rays as it fliould do^ 
according to his Calculation, which is^ as 9 to 16 ; Whereas, ac- 
cording to his T<^W^ of Apertures^ an excellent 5 5 (oozTelefcops 
flioald bear finches ^/?^r^2^ri? in proportion to excellent fmal! 
ones. He notes alfo, that the Eye-glafs of the faid T^f/^y^c/^^ , 
compofed of 2 Glaffes, hath no more efFe£t, when it is moft' 
charged, than a Glafs of 4^ inches , which makes it magnifie 
not a 100 times, And he finds by Mr. H©^?/^, that he efteems a 
Telefcope made in London of 60 feer^ (which amount to about 57 
fcQt o{ France, the f oot of Fr«^^r^ being to rhat of England as a- 
bout 15 to 16 y becaufe itcanbearatleafb 3 Enghfb inches A- 
perjure^ and that thefe are few of 10 feet, that can bear more 
than 2 inches, ( which is but zi^- Lims French) although he (Me: . 
Au^nut ) gxvts no \z{s Aperture. thdLn io^ te a 1% ioot-Telefcope^^ 
and his of 21 feet hath ordinarily 2 Inches, 4 LineSj or 2 inches^ 
^ Li s Aperture, 
This Difcourfe he Goncludeth with exhorting thofe, that 
work OpticJ{ Glaj!es, to endeavor to make them fuch , that they 
lUay bear great Aper 'xires and deep Eye-glafTesj feeing it is not 
thelc'ngth that gives eftecm ioTilefcopes > but on the contrary 
renders them Icfs eftimable^ by reafon of the trouble accom^ 
panying^ 
