CHRISTIAAN HUYGENS

it can be viewed. But here, because of the amplitude of the lunar disk, to prevent light from reaching the eye from one side, or by any other path than through the larger lens while the observer is looking at a part of it, a paper circle is placed around this lens, with a diameter a little more than double that which would cover the whole Moon. Unless this is done, the shadows and those tracts which, being darker than the others, are accustomed to be seen on its globe, appear more faint. And thus we have now explained the entire principle and apparatus of our aerial telescope, not at all laborious; and with that thread, as if it were Ariadne's, we have found a way out where previously there was none.

Furthermore, so that the things we have said may be understood more clearly, we submit the delineation here, in which:

The mast is a b.
The small board movable in the channel is c d.
The arm fixed to it at right angles is e.
The transverse rod upon which the lens is placed is f f.
The rope returning into itself is g g.
The lead weight attached to the rope is h.
The small disk at the top of the mast is a.
The hollow cylinder containing the primary lens is i.
The rod attached to the cylinder is k l.
The brass globe clinging to the rod and turnable in the underlying hollow is m.
The lead weight joined to the bronze wire is n.
The short and flexible stylus inserted at the end of the rod is l.
The small tube carrying the minor or eyepiece lens is o.
The rod attached to the tube is p.
The movable small axle is q.
The handle to be held by the hand is r.
The leaden ball is s.
The spindle around which the thread is wrapped is t.
The sights placed crosswise, thus creating a hole through which the thread is passed, are u.
The thin silk thread is l u.
The fulcrum upon which the spectator leans is x.
The lantern is y.

The triangles arranged along the mast, by which one might climb, have been omitted so as not to make the figure more obscure.