KALEIDOSCOPE. This beautiful optical instrument was invented by Dr. (Sir David) Brewster, and made public in 1817. The es sential parts of the instrument consist of two' plane mirrors of glass, having their hinder surfaces blackened in order to prevent any re flection of light from thence. Each mirror is from six to ten inches long, and of a trapezoi dal form; the larger end about an inch and a half wide, and the shorter end about three quarters of an inch ; and the two are 'placed in contact with one another at the wide end of each, so as to form a dihedral angle, the like ends being placed together. The object to be viewed is disposed contiguously to the larger ends, and the eye should be near the opposite extremity, but a little above the line of contact. The effect produced by the re flections of the light from the two surfaces is singularly beautiful ; the objects viewed undergo multiplied reflections, and assume graceful symmetrical forms. Sir David Brew ster found means to obtain multiplied images of such objects as flowers, trees, and even per sons or things in motion : and thus the im portance of the instrument was greatly increased. Some kaleidoscopes have been
executed in such a manner that the two mirrors may be placed at any required angle with one another ; by which means the images in the visible field of view may be varied at pleasure. The instrument is capable also of being constructed so that the multiplied image may be projected on a screen, and thus made visible at one time to many spectators. Again, Dr. Roget has shown that the properties of the instrument may be greatly extended by employing, instead of two, three and even four plain mirrors, united together at their edges so as to form a hollow prism, or a frustum of a pyramid, the reflecting surfaces being to wards the interior. These are called poly central kaleidoscopes.