change the Center; and that the Glass is not pressed at the same time by the Ring but in one part on the side, vid. about a fourth; and that the parts of the Glass are not equally worn away, &c. What then, saith he, would become of a Glass of 10000 feet, which, according to the said Table, would have more than four feet, or four feet and nine inches, or five feet, seven inches Aperture, and of which the Ring, though it were two feet nine inches, would have but one minut of inclination, and the Glass of 5 feet Aperture would have but 4 minuts, and the curvity of it would be less than the eight[errata 1] part of a Line.
But, saith he, let us consider, only a Glass of 300 foot, to see, what is to be hoped of that, and to know at least the difficulty, to be met with in making a Glass only of that Length. A Glass then of 300 foot, according to his Table, must have more than 8 inches Aperture, which maketh but 16 minuts of its Circle, and it should have more than 11 inches, if it be an excellent one. If Mr. Hook (adds he) did use but his Ring of 6 inches, which he would use from twelve to an hundred foot Glass, the Inclination, which the Axis, or Mandril, that bears his Glass, should have, should be but 16 minuts, and the Curvity of the Glass would be less than the eighth part of a Line, and if he should use a bigger; the Inclination would be proportionable.
Whence it may be judged (continues he) that we are yet very far from seeing Animals &c. in the Moon, as Monsieur Des Cartes gave hope, and Mr. Hook despairs not of. For, he believes by what he knows of Telescopes, that we are not to look for any above 300 or 400 foot at most; and he fears, that neither Matter nor Art will go even so far.
When therefore (saith he) a Glass of 300 foot should bear an Eye-glass of 6 inches (which would appear wonderful) it would magnifie but 600. times in Diameter, that is, 360000 times in Surface: but suppose, that such could be made, as would magnifie a 1000 times in Diameter, and 1000000. of times in Surface, admitting there were but 60000 leagues from the Earth to the 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
- Original: hundred was amended to eight: detail