If the plates are narrower at the eye-end, as in Fig. 21, the angular point =E= should be a little on one side of the axis of the tube, in order that the aperture in the centre of the brass cap next the eye may be brought as near as possible to =E=. When the plates have the same breadth at both ends, the angular point =E= will be near the lower circumference of the tube, as it is at =O=; and in this case it is necessary to place the eye-hole out of the centre, so as to be a little above the angular point =E=. This construction is less elegant than the preceding; but it has the advantage of giving more room for the introduction of a feather, or a piece of thin wood covered with leather, for the purpose of removing the dust which is constantly accumulating between the reflectors. In some instances the plates have been put together in such a manner that they may be taken out of the tube, for the purpose of being cleaned; but though this construction has its advantages, yet it requires some ingenuity to replace the reflectors with facility, and to fix them at the exact inclination which is required. One of the most convenient methods is to support the reflector in a groove cut out of a solid cylinder of dry wood of nearly the same diameter as the tube; and after a slip of wood, or any other substance, is placed along the open edges of the plates, to keep them at the proper angle given by the groove, the whole is slipped into the tube, where it remains firm and secure from all accident. If the length of the reflectors is less than the shortest distance at which the eye is capable of seeing objects with perfect distinctness, it will be necessary to place at the eye-end =E= a convex lens, whose focal length is equal to, or an inch or two greater than, the length of the reflectors. By this means the observer will see with perfect distinctness the objects placed at the object end of the Kaleidoscope. This lens, however, must be removed when the instrument is to be used by persons who are short-sighted. The proper application of the objects at the end of the reflectors is now the only step which is required to complete the simple Kaleidoscope. The method of forming, selecting, and mixing the objects, will be described in the next chapter. At present, we shall confine our attention to the various methods which may be employed in applying them to the end of the reflectors. [Illustration: FIG. 22.]