Machines for Grinding Concave Mirrors
Wheels, a bow and toothed rings that grind mirrors to true concavity; a note on alloys
The sheet gathers machines for grinding mirrors to a perfect concavity, using a wheel, a fiddle-bow drive and toothed rings that impart two or three combined circular motions so that every imperfection is worn away. The notes claim such revolutions generate two perfect planes, or a truly concave mirror even from an imperfect grinding wheel. A separate figure at the top treats a sphere that hurls a projectile, and a closing metallurgical remark warns that burnt copper or arsenic in the mixture makes it brittle. Long-armed levers span the top and bottom of the page.
On this page
Throwing figure (a, b)
With a b twenty braccia long, the sphere is said to hurl its projectile ten braccia away from itself, the motion having arisen from a circle forty braccia across. A long lever arm spans the top edge of the sheet.
Grinding a perfectly concave mirror (n, c, a, m, b)
The wheel n hollows the mirror to perfect concavity by means of the bow a, which turns the pinions b, which in turn revolve the mirror m. Even if the grinding wheel is not itself of perfect circumference, the mirror ground with it wears away every imperfection, so that two perfect bodies result.
Machine of three circular motions (m, n, a, b, r)
A principal wheel turning about the pole m drives, through an iron reaching from one elbow to the other, a mirror turning about the pole n. The mirror has two motions: it spins on itself within a toothed ring, and it revolves as it engages the ring's inner teeth, the whole grinding two perfect planes.
Vertical grindstone and a note on alloys
A toothed wheel drives a vertical grindstone, said to resemble the penultimate mode above. A metallurgical aside warns that putting burnt copper into the mixture, or corrupting it with arsenic, will make it brittle.