shap'd like this, but it has an even neck instead of a taper one, and
runs in a Collar, that by the help of a Screw and a joynt made like M
in the Figure, it can be still adjustned to the wearing or wasting
neck: into the end of this _Mandril_ is screwed a Chock N on which with
Cement or Glew is fastned the piece of Glass Q that is to be form'd;
the middle of which Glass is to be plac'd just on the edge of the Ring
and the Lath OP is to be set and fixt (by means of certain pieces and
screws the manner whereof will be sufficiently evidenc'd by the Figure)
in such an Angle as is requisite to the forming of such a Sphere as the
Glass is design'd to be of; the geometrical ground of which being
sufficiently plain, though not heeded before, I shall, for brevities
sake, pass over. This last _Mandril_ to be made (by means of the
former, or some other Wheel) to run round very swift also, by which two
cross motions the Glass cannot chuse (if care be us'd) but be wrought
into a most exactly spherical Surface.

But because we are certain, from the _Laws of refraction_ (which I I have
experimentally found to be so, by an Instrument I shall presently describe)
that _the lines of the angles of Incidence are proportionate to the lines
of the angles of Refraction_, therefore if Glasses could be made of those
kind of Figures, or some other, such as the most incomparable _Des Cartes_
has invented, and demonstrated in his Philosophical and Mathematical Works,
we might hope for a much greater perfection of Opticks then can be
rationally expected from spherical ones; for though, _cæteris paribus_, we
find, that the larger the _Telescope_ Object Glasses are, and the shorter
those of the _Microscope_, the better they magnify, yet both of them,
beside such determinate dimensions, are by certain inconveniences rendred
unuseful; for it will be exceeding _difficult_ to make and _manage_ a Tube
above an _hundred foot long_, and it will be as difficult to _inlighten_ an
Object less then an hundred part of an inch distant from the Object Glass.