kaleidoscope, optical instrument that uses mirrors to produce changing symmetrical patterns. Invented by the Scottish physicist Sir David Brewster in 1816, the device is usually a hand-held tube, a few inches to as much as twelve feet in length, and looks like a small telescope. At one end of the tube is an eyepiece; at the other end colored chips of glass are loosely sandwiched between two glass disks. Between the ends of the tube are two rectangular plane mirrors. The long edge of one of the two mirrors lies against the long edge of the other at an angle, their intersection lying close to the axis of the tube. The glass chips form patterns where they lie, and these patterns change as the chips fall into new positions when the tube rotates. Each pattern undergoes multiple reflections in the mirrors in such a way as to produce a resulting symmetrical pattern as seen through the eyepiece.

The world's largest kaleidoscope, located in Mt. Tremper, N.Y., is 64 ft (19.5 m) tall. There is no eyepiece; people stand inside the base to view the image, which is projected downward onto three reflective panels to produce a spherical cluster of 254 hexagonal facets that appears to be 50 feet across. For Expo 2005 in Aichi, Japan, a 130-ft-high (40-m) kaleidoscope was constructed in the three-sided Earth Tower; three enormous, oil-filled revolving disks filtered incoming light that was reflected by huge mirrors to produce a spherical image some 118 ft (36 m) in diameter; the image was viewed by standing inside the tower.

Optical device consisting of mirrors that reflect images of bits of coloured glass or other objects in a symmetrical geometric design through a viewer. The design may be changed endlessly by rotating the section containing the loose fragments. A simple kaleidoscope consists of two thin, wedge-shaped mirror strips touching along a common edge. The mirrors are enclosed in a tube with a viewing eyehole at one end. At the other end is a thin, flat box that can be rotated; it is made from two glass disks, the outer one ground to act as a diffusing screen. In this box are pieces of coloured glass, beads, etc. When the box is turned, the objects inside tumble into an arbitrary grouping, and when the diffusing screen is illuminated, the sixfold or eightfold multiplication creates a striking symmetrical pattern. The kaleidoscope was invented by Sir David Brewster circa 1816.

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