means of its weight, or by the means of a spring,
pressing it, without wrigling from one place to another, as it would
presently happen in the fashion, {62} as the _Turn_ is composed. For, when
the Glasses do wear, especially when they are very _convex_, it cannot be
otherwise, 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 industry
of Mr. _Hook_, considering what he saith in the _Preface_ of his
_Micrography_, touching a Method, he knows, of finding out as much in
_Mechanicks_, as can be found in _Geometry_ by _Algebra_.

Besides this, he taketh notice, that most of those that medle with
_Optick-Glasses_, give them not as much _Aperture_, nor charge them so deep
as they ought. And he instances in the _Telescope_, which His _Majesty_ of
_Great Britain_ presented the _Duke_ of _Orleans_ with, _videl._ that it
did bear but 2 inches, and 9 lines _French_, for its greatest _Aperture_,
though there be 5 or 6 lesser _Apertures_, of which it seems (_saith he_)
the Artificer would have those, that use it, serve themselves more
ordinarily, than of the greatest; which conveys but almost half as many
Rays as it should do, according to his Calculation, which is, as 9 to 16;
Whereas, according to his _Table_ of _Apertures_, an excellent 35 foot
_Telescope_ should bear 4 inches _Aperture_ in proportion to _excellent_
small ones. He notes also, that the Eye-glass of the said _Telescope_,
composed of 2 Glasses, hath no more effect, when it is most charged, than a
Glass of 41/2 inches; which makes it magnifie not a 100 times. And he finds
by Mr. _Hook_, that he esteems a _Telescope_ made in _London_ of 60 feet,
(which amount to about 57 feet of _France_, the foot of _France_ being to
that of _England_ as about 15 to 16) because it can bear at least 3
_English_ inches _Aperture_, and that there are few of 30 feet, that can
bear more than 2 inches, (which is but 221/2 Lines _French_) although he (M.
_Auzout_) gives no less _Aperture_ than so, to a 15 foot-_Telescope_, and
his of 21 feet hath ordinarily 2 Inches, 4 Lines, or 2 inches, 6 Lines
_Aperture_.

This Discourse he Concludeth with exhorting those, that work
_Optick-Glasses_, to endeavor to make them such, that they may bear great
_Apertures_ and deep Eye-glasses; seeing it is not the length that gives
esteem to _Telescopes_; but on the contrary renders them less estimable, by
reason of th