as the Turn is compofed. For, when the GlafTes do wear , 
fpecially 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 leavs 
to the induftry of Mr. Hoof, confidering what he faith in the 
Preface of his Micrography, touchinga Method, he knows, of 
finding out as much in Mechanicpj. as can be found in Geometry 
by Algebra , . 
Befides this, he taketh notice, that mo ft of thofe that medle 
with Qptic\-GIa£ef, give them not as much Aperture , nor. charge 
them (o deep as they ought. And he inftanCes in the Tele [cope , 
which His Majejly of Great Britain prefented the Dube of Or- 
leans with, videl. that it did bear but 2 inches, and 9 lines French 
for its greateft Aperture , though there be 5 or 6 leffer Aptrtures r 
of which it feems (faith he) the Artificer would have thofe, 
that ufe it, ferve themfelves more ordinarily, than of the great- 
efts which conveys but almoft half as many Rays as it fhould do, 
according to his Calculation, which is, as 9 to 16 j Whereas, ac- 
cording to his Table of Apertures, an excellent 35 foot Tele [cope 
fhould bear ^4 inches Aperture in proportion to excellent final! 
ones. He notes alfo, that the Eye=glafs of thefaid Telefcope, 
compofed of 2 GlafTes, hath no more effedfc, when it is moft 
charged, than a Glafs of 4^ inches 3 which makes it magnifie 
not a 100 times. And he finds by Mr. Hoop, that he efteems a 
Telefcope mz&Q in London of do feet, (which amount to about 57- 
feet of France, the foot ot France being to that of England as a- 
bout 15 to 1 5 ) becaufe it can bear atleaft 3 Enghfh °\ nches A- 
perture, and that there are few of 30 feet, that can bear more 
than 2 inches, ( which is but 22- Lines French) although he (M. 
Au^out ) gives no lefs Aperture, than fo, to a 15 foo i-Telefcope 3 
and his of 2 % feet hath- ordinarily 2 Inches, 4 Lines, or 2 inches, 
d Lines Aperture. 
This Difcourfe he Concludeth with exhorting thofe, that 
work Opticf Glafes, to endeavor to make them fuch , that they 
may bear great Apertures and deep Eye-gfafiesj feeing it is not 
the length that gives efteem to Telefcopes s but on the contrary 
Benders them left eftimable, by reafon of the trouble accom- 
; - V P an y in §- 
