Kaleidoscope (Gr. beautiful, a form, and to see), an optical instrument for multiplying the reflected images of small colored objects, producing by the symmetry of their arrangement patterns of great beauty. An instrument on this principle was originally described by Battista della Porta and Kircher; and in a work by R. Bradley, published in 1717, entitled "New Improvement of Planting and Gardening," it was recommended for aiding in the production of designs for garden plots and fortifications. Its true principles were first developed, however, by Sir David Brewster, who devised the proper method of its construction, and in 1817 took out a patent for it. - When two oblong mirrors of the same dimensions are placed so as to hinge together along an edge of each, their reflecting surfaces facing each other, and are then opened, so as to make an angle which is an aliquot part of 180°, an object placed between the planes of the mirrors, or in contact with one of the extremities of the pair, is reflected from one mirror to the other, and produces as many images as the angle of the opening is contained in 360°. These images are arranged in symmetrical order around a circular area, the radius of which is the width of the mirror, and the centre the point of meeting in the two planes.

The perfect symmetry of their arrangement depends on the angle of the opening being an aliquot part of two right angles, and that usually employed is either 18° or 20°. Another requisite is, that the line of junction of the two mirrors should be fine and smooth, as any irregularities would produce imperfections. As usually constructed, the mirrors are strips of glass blackened on one side. They are kept together by a piece of cloth glued over the edges in contact, and the proper angle is preserved by securing them in a tube of suitable shape. The open side of the triangular prism formed by the two mirrors is closed by a strip of black velvet of suitable width glued to the backs of the two mirrors. The cylindrical tube is of the diameter of the larger end of the prism, and the angle formed by the meeting of the two planes at the other extremity is nearly coincident with the centre of the circular end of the tube. Through the cover of this a small aperture is made exactly in the angle, to which the eye is to be applied in using the instrument. At the other extremity a plain disk of thin transparent glass is fitted close to the ends of the mirrors, and outside of this is another disk, the two kept apart by a ring set in between them.

In the intervening space the objects to be reflected are placed. These may be small fragments of colored transparent glass intermixed with a variety of other small bright objects. But care must be taken not to fill the case too full for the objects to move freely among themselves while the tube is made to turn in the hand upon its axis. By looking into the circular aperture made for the eye, the most gorgeous figures are perceived symmetrically arranged, and all forming one complete pattern. - Kaleidoscopes are also made with three, four, five, or more mirrors, and are then termed polycentral. To produce symmetry and regularity of form in the images of these kaleidoscopes, the angles which the mirrors make with each other must necessarily be aliquot parts of 180°; and as their number is increased, the range of the instrument in the variation of these angles is diminished. Thus three mirrors only should be arranged to make the three angles of 60° each, or two of 45° each and one of 90°, or one of 30°, one of 60°, and one of 90°. By the first arrangement, the images appear in groups of three repeated throughout the pattern. This instrument is called the triascope.

By the second arrangement, the instrument, called the tetrascope, produces a pattern divided into square compartments. By the third arrangement, the pattern, of hexagonal form, presents a remarkable symmetry, and the instrument is termed a hex-ascope. The last two forms are especially useful to the draughtsman.