POLYSCOPE

, or Polyhedron, in Optics, is a multiplying glass, being a glass or lens which represents a single object to the eye as if it were many. It consists of several plane surfaces, disposed into a convex form, through every one of which the object is seen.

Phenomena of the Polyscope.—1. If several rays, as EF, AB, CD, fall parallel on the surface of a Poly- scope, they will continue parallel after refraction. If then the Polyscope be supposed regular, LH, HI, IM will be as tangents cutting the spherical convex lens in F, B, and D; and consequently, rays falling on the points of contact, intersect the axis. Wherefore, since the rest are parallel to these, they will also mutually intersect each other in G.

Hence, if the eye be placed where parallel rays decussate, rays of the same object will be propagated to it still parallel from the several sides of the glass. Wherefore, since the crystalline humour, by its convexity, unites parallel rays, the rays will be united in as many different points of the retina, a, b, c, as the glass has sides. Consequently the eye, through a Polyscope, sees the object repeated as many times as there are sides. And hence, since rays coming from very remote ohjects are parallel, a remote object is seen through a Polyscope as often repeated as that has sides.

2. If rays AB, AC, AD, coming from a radiant point A, fall on several sides of a regular Polyscope;| after refraction they will decussate in G, and proceed on a little diverging.

Hence, if the eye be placed where the rays decussate after coming from the several planes, the rays will be propagated to it from the several planes a little diverging, or as if they proceeded from different points. But since the crystalline humour, by its convexity, collects rays from several points into the same point; the rays will be united in as many different points of the retina, a, b, c, as the glass has sides; and consequently the eye, being placed in the focus G, will see even a near object through the Polyscope as often repeated as that has sides.

Thus may the images of objects be multiplied in a camera obscura, by placing a Polyscope at its aperture, and adding a convex lens at a due distance from it. And it makes a very pleafant appearance, if a prism be applied so that the coloured rays of the sun refracted from it be received on the Polyscope: for by this means they will be thrown on a paper or wall near at hand in little lucid specks, much exceeding the brightness of any precious stone; and in the focus of the Polyscope, where the rays decussate (for in this experiment they are received on the convex side) will be a star of surprising lustre.

Farther, if images be painted in water-colours in the areolæ or little squares of a Polyscope, and the glass be applied to the aperture of a camera obscura; the sun's rays, passing through it, will carry with them the images, and project them on the opposite wall.—This artisice bears a resemblance to that other, by which an image on paper is projected on the camera; viz, by wetting the paper with oil, and straining it tight in a frame; then applying it to the aperture of the camera obscura, so that the rays of a candle may pass through it upon the Polyscope.

To make an Anamorphosis, or Deformed Image, which shall appear regular and beautiful through a Polyscope, or Multiplying Glass.—At one end of a horizontal table erect another perpendicularly, upon which a figure may be designed; and on the other end erect another, to ferve as a fulcrum or support, moveable on the horizontal one. To the fulcrum apply a plano-convex Polyfcope, consisting, for example, of 24 plane triangles; and let the Polyscope be fitted in a draw-tube, of which that end towards the eye may have only a very small aperture, and a little farther off than the focus. Remove the fulcrum from the other perpendicular table, till it be out of the distance of the focus; and the more so, as the image is to be greater. Before the little aperture place a lamp; and trace the luminous areolæ projected from the sides of the Polyscope, with a black lead pencil, on the vertical plane, or a paper applied upon it.

In the several areolæ, design the different parts of an image, in such a manner as that, when joined together, they may make one whole, looking every now and then through the tube to guide and correct the colours, and to see that the several parts match and fit well together. As to the intermediate space, it may be filled up with any figures or designs at pleasure, contriving it so, as that to the naked eye the whole may exhibit some appearance very different from that intended to appear through the Polyscope.

The eye, now looking through the small aperture of the tube, will see the several parts and members dispersed among the areolæ to exhibit one continued image, all the intermediate parts disappearing. See ANAMORPHOSIS.