A toy invented by Sir David Brewster (1781-1868) consisting of a cylinder with two mirrors inside and an eyepiece at one end.

Sometimes the other end looks out at the world, and sometimes there is a container with coloured objects, such as small pieces of glass, in there. Usually the container can be rotated. Looking in the eyehole you'll see a pattern with hexagonal symmetry.

Small (and not so small) children can be amused for hours by this. Everyone should own at least one kaleidoscope. You can find much more information on the web, of course. One good starting point is Kaleidoscope Heaven.