POLYOPTRUM

, in Optics, a glass through which objects appear multiplied, but diminished. The Polyoptrum differs both in structure and phenomena from the common multiplying glasses called Polyhedra or Polyscopes.

To construct the Polyoptrum.—From a glass AB, plane on both sides, and about 3 fingers thick, cut out spherical segments, scarce a 5th part of a digit in diameter.—If then the glass be removed to such a distance from the eye, that you can take in all the cavities at one view, you will see the same object, as if through so many several concave glasses as there are cavities, and all exceeding small—Fit this, as an objectglass, in a tube ABCD, whose aperture AB is equal to the diameter of the glass, and the other CD is equal to that of an eye-glass, as for instance about a finger's breadth. The length of the tube AC is to be accommodated to the object-glass and eye-glass, by trial. In CD fit a convex eye-glass, or in its stead a meniscus having the diftance of its principal focus a little larger than the length of the tube; so that the point from which the rays diverge after refraction in the objectglass, may be in the focus. If then the eye be applied near the eye-glass, a single object will be seen repeated as often as there are cavities in the object-glass, but still diminished.