KALEIDOSCOPE, an instrument which produces a sym metrical set of images, by the to and fro reflections in a pair of plane mirrors meeting one another at an angle, which is an even submultiple of 36o°. It was invented by Sir David Brewster, and patented by him in 1817. An attempt was made to discount the invention as being merely the same as the multiple mirrors of Kircher, Harris, Wood, Bradley and others; but none of these writers had described an arrangement that produced the beauti fully symmetrical patterns that resulted from Brewster's design.
Essentially the instrument consists of two flat pieces of glass, with the edge of one surface meeting the other surface in a clean line; and with their planes inclined to one another at some such angle as 3o°(360\ or eye of the observer is k 12 applied at one end, close to the line of intersection of these glasses, and near the opposite end is placed any suitable object in the angular space between the mirrors. The backs of the glasses are blacked to cut off any reflection from them ; or, preferably, the inner surfaces of the glasses may be silvered, or metal mirrors may be substituted. The mirrors are usually mounted in a tube, and they are cut somewhat tapering at the eye end, so that their line of intersection may conveniently be brought towards the centre of the tube, and the eye thus be able to look through an aperture central with the tube. The objects usually supplied are a num ber of pieces of coloured glass, mounted in a cell between a disc of clear glass, which is close to the end of the mirrors, and an outer disc of ground glass. On rotating this cell the pieces of glass fall about and form an infinite variety of patterns. Much more delightful effects are produced by a further device of Brew ster. He mounted a convex lens of short focal length (two or three inches) in a sliding tube, which could then be adjusted to throw the image of any natural object in the plane of the ends of the mirrors.
The field of view is limited by the edges of the mirrors opposite to the eye. If, as is usually the case, the object is at a little distance from the end, unless the eye can be placed absolutely in the plane of the mirror, the edge of the mirror will cut the object and the image in different lines, and they will not appear to be perfectly symmetrical. A plane object, e.g., a drawing, which is placed close against the far edges of the mirrors, or a solid object which is placed actually within the angle of the mir rors, and nearer the eye than the ends of the mirrors, will form a symmetrical pattern, independently of the position of the eye.
Or if the eye could be placed in the line of intersection of the mirrors, so that it is in the plane of both mirrors, the pattern would always be symmetrical, even if the object is beyond the far end of the mirrors. In actual practice, the eye is arranged to be placed as close as possible to the angle of the mirrors, and the objects are also kept close to the ends of the mirrors. The pat tern will then not be quite perfect, but will be sufficiently sym metrical to satisfy an ordinary observer, whose attention has not been specially directed to the want of symmetry.
From the remarks in the last paragraph, it is obvious that it is possible to form a perfectly symmetrical picture of a solid object, which can be seen in relief by the two eyes simultaneously. It is only necessary that the object shall be placed within the far ends of the mirrors, and if the latter are of sufficient size, it can be viewed by both eyes and appear symmetrical, even though the eyes are not close to the angle of the mirrors. A bunched-up handkerchief, or a coloured piece of silk, lighted strongly from one side, yields most beautiful designs, which look like exquisite exotic flowers.
The kaleidoscope is frequently used by designers to enable them to realise the effects that can be produced by symmetrical designs. In this case the mirrors are usually set up in a vertical plane, and supported a little above a table, on which any object can be placed.
Attempts have often been made to project kaleidoscopic pat terns, but usually with indifferent success. If a pair of mirrors are merely interposed between the condenser and the projecting lens, it will be found impossible to illuminate the images formed by the successive reflections. A piece of ground glass placed immediately after the condenser will help to make the lighting more even, but at a great sacrifice of light. As many beams of light as there are reflected images are required, one beam being focussed upon each of the images of the projecting lens. This can be obtained by interposing a second pair of mirrors between the arc and the condenser, these mirrors making the same angle with one another as the front pair. The lines of intersection of each pair of mirrors and the axis of the condenser should all be coincident. A group of images of the arc arranged round a circle, will then be formed, and the condenser should focus these on the centres of the set of images of the projecting lens. (R. S. CO