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 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 inconveniences 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 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,