_Moon_, and that the smalness of the _Aperture_ of the
Glasses (which yet would diminish the Light more than 36 times) and the
obstacle of the Air were not considered, we should not {61} see the _Moon_,
but as if we were a 100, or at least, 60. leagues distant from her without
a Glass. He here wishes, that those, that promise to make us see _Animals_
and _Plants_ in the Moon, had thought on what our naked Eyes can make us
discern of such Objects, only at 10 or 12 leagues distance.

But this he would not have understood as a discouragement from searching
with all care and earnestness after the means of making long _Telescopes_,
or of facilitating the working thereof; but only as an Advertisement to
those, who light upon the _Theory_ of any _Engine_, not to expose it
presently as possible and useful, before they have tried it, or if it have
succeeded in small, not to endeavour to persuade, that it will also succeed
in great.

As it may happen (_saith he_) that the Engin of Mr. _Hook_ may, by using
all necessary precautions, succeed in the making of _Eye-Glasses_, or
_small_ Optick-Glasses, but not in making _great_ ones; as we see, that an
instrument composed of two Rulers, wherewith are traced Portions of
Circles, succeeds well enough in _small_, but when there is no more than
half a Line, a quarter of a Line, or less convexity, it will be no longer
just at all, as he tells us to have made the proof of it in Circles drawn
by the means of one of these Instruments, made by one of the best Workmen
in his time, who, whilst he lived, esteemed them above price, although they
be not just; as others and my self (_saith he_) have by tryal found, when
we endeavoured to make _Moulds_ by their means, & as those, who by the like
Instrument laboured to trace portions of Circles of 80 or 100 foot, _&c.
Diameter_, can attest.

But, notwithstanding all this, he hath thought upon two or three things,
which he thinks may remedy some inconveniencies of Mr. _Hook_ his _Turn_.
The _first_ is, to invert the Glass, and to put it under the _Ring_, that
so not only the Glass may be placed more _Horizontally_, and not slide upon
the _Cement_, but that the _Sand_ also, and the _Putty_ may stay upon the
Glass.

The _other_ is, that there must be two _Poppetheads_, into which the
_Mandril_ must pass, where the _Ring_ is to be fastned; and the _Mandril_
must be perfectly _Cylindrical_, that so it may advance upon the Glass as
it wears away by the