(543)
the other two Thirds of it, by at least as much as the depth of the small Screw made on it: The thread of the Screw of the bigger Third is as small, again, as that of the Screw of the other two Thirds. To the grosser Screw is adapted a Socket (f) fasten'd to a long Bar or Bolt (g g), upon which is fasten'd the moveable Sight (h), so that every turn of the Screw promotes the Sight (h) either a thread nearer, or a thread farther off from the fixt Sight (i). The Bar (g g) is made exactly equal and fitted into two small Staples (k k), which will not admit of any shaking. There are 60 of these threads; and answerable thereto, are made 60 divisions on the edge of the Bolt or Ruler (g g); and a small Index (l) fixt to the Box (a a a) denotes, how many threads the edges of the two Sights (h) and (i) are distant; and the Index (e e) shews on the Circular Plate what part of a Revolution there is more, every Revolution, as was said before, being divided into 100 parts. At the same time that the moveable Sight (h) is moved forwards or backwards, or more threads of the courser Screw, is the Plate (p p. in Fig. 2.) by the means of the Socket (q) to which it is screwed, moved forward or backward, or more threads of the finer Screw: So that this Plate, being fixt to the Telescope by the Screws (r r. in Fig. 2.) so as the middle betwixt the Sights may lye in the Axis of the Glass, however the Screw be turn'd, the midst betwixt the Sights will always be in the Axis, and the Sights will equally either open from it, or shut towards it.
Fig. 2. represents the moveable Cover containing the Screws, to be by the Bookseller cut off by the pricked Line (x x x) from the Paper, and to be fitly placed on Figure 1. according to the pricked Line (y y y) answering thereto; that by the taking off, as it were, or folding up of this Cover, the inward contrivance of the Screws and Sights may appear.
And because it is conceived by some ingenious Men, that it will be more convenient, instead of the Edges of the two Sights (h and i) to employ two Sights fitted with Hairs, therefore is added Figure 3. representing the two Sights (r and s) so fitted with Threads (t and u) that they may be conveniently used in the place of the solid Edges of the Sights (h and i).
The fourth Figure represents, How the Screws are to be put on.
