polyhedron
A polyscope, or multiplying glass.
polyhedron: multi-faceted lens that splits one image into many
A polyhedron is an optical element with multiple flat or slightly curved facets ground into a single piece of glass or transparent material. Each facet acts as an independent lens, all aligned to view the same object from slightly different angles. When you look through a polyhedron, you see a regular array of identical images rather than a single view. The device multiplies and replicates what lies in front of it.
The facets are typically arranged in a square or hexagonal grid pattern across the glass surface. Each facet may be between 2 and 10 millimeters in diameter, depending on the intended magnification and image separation. The facets can be flat (which produce no magnification) or gently curved (which magnify each individual image). The angle and spacing of the facets determines how far apart the replicated images will appear to the viewer.
Polyhedrons are used in precision measurement and optical quality control. An inspector looking through one can compare multiple views of a surface texture or dimensional feature simultaneously, making it easier to spot variations in finish or alignment. They also appear in certain types of magnifying glasses intended for detailed work, where the multiple images help the user confirm that they are looking at the correct region or angle.
The name comes from the Greek roots for many and face, referring directly to the multiple faceted surfaces. The older term polyscope is less common in modern usage but refers to the same optical principle. The device should not be confused with a true polyhedron in geometry, which is a solid body bounded by plane faces; the optical element is merely ground with a faceted surface.
Manufacturing a polyhedron requires precision grinding and polishing to hold the facet spacing to tight tolerances, usually within a fraction of a millimeter. Any deviation in facet angle or spacing causes the replicated images to misalign or blur. The glass must be optically clear with minimal internal stress, since defects will be magnified across all replicated views. These elements are custom-made for specific applications rather than mass-produced.