The objects which give the finest outlines by inversion, are those which have a curvilineal form, such as circles, ellipses, looped curves like the figure 8, curves like the figure 3 and the letter S; spirals, and other forms, such as squares, rectangles, and triangles, may be applied with advantage. Glass, both spun and twisted, and of all colours, and shades of colours, should be formed into the preceding shapes; and when these are mixed with pieces of flat coloured glass, blue vitriol, native sulphur, yellow orpiment, differently coloured fluids, enclosed and moving in small vessels of glass, etc., they will make the finest transparent objects for the Kaleidoscope. When the objects are to be laid upon a mirror plate, fragments of opaquely-coloured glass should be added to the transparent fragments, along with pieces of brass wire, of coloured foils, and grains of spelter. In selecting transparent objects, the greatest care must be taken to reject fragments of opaque glass, and dark colours that do not transmit much light; and all the pieces of spun glass, or coloured plates, should be as thin as possible. When the objects are thus prepared, the next step is to place them in the object-box. The distance between the interior surfaces of the two plane glasses, of which the object-plates are generally composed, should be as small as possible, not exceeding ⅛th of an inch. The outermost of these glasses has its external surface rough ground, or is what is called a _grey glass_, the principal use of which is to prevent the lines of external objects, such as the bars of the window, or the outlines of the illuminating flame from being introduced into the picture. When a strong light is used, a circular disk of fine thin paper placed outside of the object-box may be advantageously employed in place of the ground glass. The thickness of the transparent glass plate next the reflectors should be just sufficient to keep the glass from breaking; and the interior diameter of the brass rings into which the transparent and the grey or ground plates of glass are burnished, should be so great that no part of the brass rim may be opposite the angular part of the reflectors during the rotatory motion of the cell. If this precaution is not attended to, the central part of the pattern, where the development of new forms is generally the most beautiful, will be entirely obliterated by the interposition of the brass rim. Instead of using transparent or grey glasses on the sides of every object-box, some of the boxes should be made with disks of flint glass, the interior surface of which have been stamped while in a state of fusion with a sort of pattern, or with curved lines of a pleasing form. In others, the outer surface alone of the plate next the reflectors might be thus formed. An object-box might also be formed of disks of glass, one side of which is colourless, and the other coloured, some of the coloured portions being ground away irregularly,