In the various forms of the Kaleidoscope which have been described in the preceding chapters, the pictures which it creates are visible only to one person at a time; but it is by no means difficult to fit it up in such a manner as to exhibit them upon a wall to any number of spectators. The necessary limitation of the aperture at the eye-end of the instrument, however, is hostile to this species of exhibition, as it requires a very intense light for the purpose of illuminating the objects. The general principle of the apparatus requisite for this purpose is shown in Fig. 50, where =C D G F= is the tube containing the reflectors =A O E=, etc. The objects from which the pictures are to be created are placed in the cell =C D=, which may be made either to have a rotatory movement round the axis of the tube, or to slide through a groove, like the sliders of a magic lantern. These objects are powerfully illuminated by a lens =B=, which concentrates upon them the direct light of the lamp or candle =H=, and also the part of the light which is reflected from the mirror =M N=. At the eye-end =E= of the Kaleidoscope, is placed a lens =L L=, close to the end of the reflectors, and having its centre coincident with the centre of the aperture at =E=. In order that this lens may form behind it an image =P P= of the objects placed in the object-plate =C D=, its focal length must be less than the length =A E= of the plates. If the focal length of =L L= is so small as one-half of =A E=, then it follows, from the principles of optics, that the distance =L P= at which the image is formed behind the lens, will be precisely equal to the distance =A E= of the object; but this is obviously too small a distance, for the diameter of =P P= would be equal only to the apparent diameter of the circular aperture of the Kaleidoscope, or to twice =A O=. Hence it is necessary, that the focal length of the lens =L L= be less than =A E=, and greater than half of =A E=. Two-thirds, or three-fourths of =A E= will be found to be a suitable focal length; for if it is larger than this, the image will be formed upon the wall or screen at too great a distance from the instrument. When the instrument is thus fitted up, an enlarged image of the pattern will be thrown upon the wall, which must be covered with white paper, or some white ground, in order to exhibit the colours to advantage. By turning the object-plate round its centre, or, if it is a rectilineal one, by pushing it through the groove, and at the same time giving it a rotatory motion, the pattern on the wall will undergo every possible transformation, and exhibit to the spectators, in a magnified form, all those variations which have been observed by applying the eye to the Kaleidoscope.