about six Inches and a quarter. The Eye-glass was Plano-convex, and the
diameter of the Sphere to which the convex side was ground was about 1/5
of an Inch, or a little less, and by consequence it magnified between 30
and 40 times. By another way of measuring I found that it magnified
about 35 times. The concave Metal bore an Aperture of an Inch and a
third part; but the Aperture was limited not by an opake Circle,
covering the Limb of the Metal round about, but by an opake Circle
placed between the Eyeglass and the Eye, and perforated in the middle
with a little round hole for the Rays to pass through to the Eye. For
this Circle by being placed here, stopp'd much of the erroneous Light,
which otherwise would have disturbed the Vision. By comparing it with a
pretty good Perspective of four Feet in length, made with a concave
Eye-glass, I could read at a greater distance with my own Instrument
than with the Glass. Yet Objects appeared much darker in it than in the
Glass, and that partly because more Light was lost by Reflexion in the
Metal, than by Refraction in the Glass, and partly because my Instrument
was overcharged. Had it magnified but 30 or 25 times, it would have made
the Object appear more brisk and pleasant. Two of these I made about 16
Years ago, and have one of them still by me, by which I can prove the
truth of what I write. Yet it is not so good as at the first. For the
concave has been divers times tarnished and cleared again, by rubbing
it with very soft Leather. When I made these an Artist in _London_
undertook to imitate it; but using another way of polishing them than I
did, he fell much short of what I had attained to, as I afterwards
understood by discoursing the Under-workman he had employed. The Polish
I used was in this manner. I had two round Copper Plates, each six
Inches in Diameter, the one convex, the other concave, ground very true
to one another. On the convex I ground the Object-Metal or Concave which
was to be polish'd, 'till it had taken the Figure of the Convex and was
ready for a Polish. Then I pitched over the convex very thinly, by
dropping melted Pitch upon it, and warming it to keep the Pitch soft,
whilst I ground it with the concave Copper wetted to make it spread
eavenly all over the convex. Thus by working it well I made it as thin
as a Groat, and after the convex was cold I ground it again to give it
as true a Figure as I could. Then I took Putty which I had made very
fine by washing it from all its grosser Particles, and laying a little
of this upon the Pitch, I ground it upon the Pitch with the concave
Copper, till it had done making a Noise; and then upon the Pitch I
ground the Object-Metal with a brisk motion, for about two or three
Minutes of time, leaning hard upon it. Then I put fresh Putty upon the
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