In constructing a Kaleidoscope upon this principle, we must procure a piece of glass entirely free from veins, and cut it into the form shown in [Fig. 49].[7] The two surfaces B O E, A O E, must be inclined at an angle which is the even aliquot part of a circle. They must be ground perfectly flat and highly polished, and the junction O E must be made as fine as possible. The upper surface A B E should be rough-ground, and the side A B O, and the side at E, should be parallel and well polished. If the glass is colourless and good, the eye, when placed at E, will see the very same appearance as in the simple Kaleidoscope; and objects placed at A B O will be arranged into the same beautiful figures. The only defects attending this form of the Kaleidoscope, are the loss of light occasioned by its passing through a mass of solid glass, not perfectly transparent, and the difficulty of obtaining a perfect junction of the two reflecting planes. The first of these evils is, however, counterbalanced by the great intensity of the light which suffers total reflexion; and the second does not exist when the Kaleidoscope is intended to give rectilineal or annular patterns.
In the construction of instruments of this kind, it is necessary to make the prism of glass longer than the distance at which the eye can see objects with perfect distinctness; that is, if the eye is capable of seeing objects distinctly at the distance of five inches, it will not perceive the same objects distinctly when they are placed at the end of a prism of glass five inches long. This singular effect arises from a property of plain lenses or pieces of plain glass, in consequence of which, they cause divergent rays to diverge from a point nearer the lens or plate, than that from which they radiated. It will therefore be more convenient, for many reasons, to make the glass prism only two or three inches long, and obtain distinct vision by means of a lens placed at the eye-end of it; but, for the reason already mentioned, the focal length of the lens must be less than the length of the glass prism. The lens may even be joined to the prism, by grinding the eye-end into a spherical form, but the degree of convexity must be calculated upon the principles already stated.
The solid form of the Kaleidoscope is peculiarly fitted for polycentral instruments, as we have only to polish the side, which would otherwise have been left rough, the prism being supposed to be cut to the angles which are necessary to give symmetrical forms, according to the principles stated in [Chapter XIII].
CHAPTER XV.
ON THE APPLICATION OF THE KALEIDOSCOPE TO
THE MAGIC LANTERN, SOLAR MICROSCOPE,
AND CAMERA OBSCURA.
Fig. 50.
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.