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 the trouble {63} accompanying them, if they perform no more, than shorter ones. Where, by the by, he takes notice, that he knows not yet, what _Aperture_ Signor _Campani_ gives to his Glasses, seeing he hath as yet signified nothing of it; but that the small one, sent by him to Cardinal _Antonio_, hath no more _Aperture_, than ordinary ones ought to have.