thing upon
the Pitch, to keep it moist without laying on any more fresh Putty. The
Object-Metal was two Inches broad, and about one third part of an Inch
thick, to keep it from bending. I had two of these Metals, and when I
had polished them both, I tried which was best, and ground the other
again, to see if I could make it better than that which I kept. And thus
by many Trials I learn'd the way of polishing, till I made those two
reflecting Perspectives I spake of above. For this Art of polishing will
be better learn'd by repeated Practice than by my Description. Before I
ground the Object-Metal on the Pitch, I always ground the Putty on it
with the concave Copper, till it had done making a noise, because if the
Particles of the Putty were not by this means made to stick fast in the
Pitch, they would by rolling up and down grate and fret the Object-Metal
and fill it full of little holes.

But because Metal is more difficult to polish than Glass, and is
afterwards very apt to be spoiled by tarnishing, and reflects not so
much Light as Glass quick-silver'd over does: I would propound to use
instead of the Metal, a Glass ground concave on the foreside, and as
much convex on the backside, and quick-silver'd over on the convex side.
The Glass must be every where of the same thickness exactly. Otherwise
it will make Objects look colour'd and indistinct. By such a Glass I
tried about five or six Years ago to make a reflecting Telescope of four
Feet in length to magnify about 150 times, and I satisfied my self that
there wants nothing but a good Artist to bring the Design to
perfection. For the Glass being wrought by one of our _London_ Artists
after such a manner as they grind Glasses for Telescopes, though it
seemed as well wrought as the Object-glasses use to be, yet when it was
quick-silver'd, the Reflexion discovered innumerable Inequalities all
over the Glass. And by reason of these Inequalities, Objects appeared
indistinct in this Instrument. For the Errors of reflected Rays caused
by any Inequality of the Glass, are about six times greater than the
Errors of refracted Rays caused by the like Inequalities. Yet by this
Experiment I satisfied my self that the Reflexion on the concave side of
the Glass, which I feared would disturb the Vision, did no sensible
prejudice to it, and by consequence that nothing is wanting to perfect
these Telescopes, but good Workmen who can grind and polish Glasses
truly spherical. An Object-