Flat-convex lenses with multi-faceted surface and ebony case

5.6 (diameter)
Glass and ebony

7 (diameter)
Glass and ebony

INDEX 1788 : V.IV.339

Vitra duo polygona ligneis thecis conclusa, quae vitra circellis laminarum Schalae opticae(n.º 320) quoque aptantur.

Two polygonal glasses enclosed in wooden cases. These glasses are also adapted to the optical scale (n.º 320) with small arched blades.

INDEX 1788 : V.IV.339

Aliud vitrum polygonum maius in simili theca.

Another larger polygonal glass in a similar case.

The two flat-convex lenses displayed are kept in cases made of ebony, with lids that screw on and off, one on each side of the ring that contains the lens.

The dimensions of the ring where the larger lens is kept as well as the screws by which the lids are fixed, are suitable for its use in any of the lens-cases of the optical bench (see instrument 90).

These lenses feature a convex multi-faceted surface, on which small squares can be seen. This detail allows for a beam of light directed perpendicularly onto the flat face of the lens, to cross it and then to fall, according to the different angles, on the surfaces of the various squares on the convex face. In this way, the different beams of light that emerge from the lens, striking a target, give rise to a discontinuous distribution of points of light, each one corresponding to one of the squares on the convex face of the lens. A beautiful effect of the dispersion of light is thus obtained.

From Colégio dos Nobres, catalogue n.º 329 and 330, respectively.

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