19 (lens diameter) 46 (total height) Ebony and glass
INDEX 1788 : V.I.331
Lens similis, aqua lucis radii longius in focum colliguntur uplici annulo ligneo excepta, ut in orbem moveri possit, pedi insistens.
Similar lens, inside of a small wooden ring, by means of which rays of light converge at a further focus; which is supported on a stand so that it can move on the axle.
|
This accessory consists of a flat-convex lens, 19 cm in diameter and with 98 cm of focal distance (or an optical power approximately equal to 1 dioptric). The lens is mounted on a wooden support, capable of turning around its horizontal axis. The wooden hoop which surrounds the lens possesses a piece in brass, on the underside, ending in a small cylindrical piece which adapts to an orifice located on the support column. This system allows for the hoop and the lens to turn around a vertical axis.
From Colégio dos Nobres, catalogue n.º 321.
's Gravesande, William Jacob, Physices Elementa, Leiden, 1742, § 3565, Tab. CXV, fig.2.
|