CHAPTER XV.
ON THE APPLICATION OF THE KALEIDOSCOPE TOTHE MAGIC LANTERN, SOLAR MICROSCOPE,AND CAMERA OBSCURA.
Fig. 50.
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 inFig. 50, whereC D G Fis the tube containing the reflectorsA O E, etc. The objects from which the pictures are to be created are placed in the cellC 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 lensB, which concentrates upon them the direct light of the lamp or candleH, and also the part of the light which is reflected from the mirrorM N. At the eye-endEof the Kaleidoscope, is placed a lensL L, close to the end of the reflectors, and having its centre coincident with the centre of the aperture atE. In order that this lens may form behind it an imageP Pof the objects placed in the object-plateC D, its focal length must be less than the lengthA Eof the plates. If the focal length ofL Lis so small as one-half ofA E, then it follows, from the principles of optics, that the distanceL Pat which the image is formed behind the lens, will be precisely equal to the distanceA Eof the object; but this is obviously too small a distance, for the diameter ofP Pwould be equal only to the apparent diameter of the circular aperture of the Kaleidoscope, or to twiceA O. Hence it is necessary, that the focal length of the lensL Lbe less thanA E, and greater than half ofA E. Two-thirds, or three-fourths ofA Ewill 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, allthose variations which have been observed by applying the eye to the Kaleidoscope.
When the preceding apparatus is used in daylight, so that the objects are illuminated by the rays of the sun, the mirrorM Nis unnecessary. The Kaleidoscope, etc., must, however, be attached to the part of the frame of a Solar Microscope, which goes into the aperture in the window-shutter.
As the most brilliant light is obtained from the burning of oxygen, either by itself, or along with coal gas, as in the Bude light,[8]a lamp of this kind is peculiarly fitted for displaying the pictures of the Kaleidoscope to a number of spectators. One of Mr. Bate’s Polycentral Kaleidoscopes was, many years ago, fitted up with a lamp of this kind, for exhibition, at the lectures on natural philosophy, delivered at Guy’s Hospital, by that eminent chemist, the late William Allen, Esq., F.R.S.
The patterns which are created by the Telescopic Kaleidoscope from natural objects, or from objects independent of the instrument, may, in like manner, be exhibited to several spectators at once. If the objects are in a room, such as bunches of flowers, statues, human figures, or large pictures, they must be placed in one apartment, and strongly illuminated. The lens must then be placed upon the endA Oof the Kaleidoscope, the object-plateC Dhaving been removed, and must be so adjusted that the images of the objects may fall exactly upon the endA Oof the reflectors. The objects may be placed at any distance from the lens, from six inches to twelve feet, accordingto their magnitude, and the pictures will be exhibited with great distinctness and effect upon the wall of the other apartment. If a blazing fire is employed, the most brilliant display of fireworks may be exhibited. When the objects are out of doors, such as trees, shrubs, etc., the Kaleidoscope, with its two lenses, must be fixed in the circular opening of a window-shutter, and the picture received upon white paper, or any other suitable ground, as in the Camera Obscura.
Similar effects may be produced in a portable Camera Obscura, by placing the apparatusC F L L G Din the moveable drawer of that instrument. If the lensL Lis of such a focal length as to admit the formation of the image within the instrument, the picture will be finely displayed upon the ground glass, and may be copied with considerable exactness.
In the preceding applications of the Kaleidoscope, the great difficulty to be overcome arises from the smallness of the aperture which can be obtained at the eye-end of the reflectors. If we take a larger aperture, for the purpose of gaining more light, the light of the reflected images is diminished by this very circumstance, and the picture loses its symmetry at the centre. The only method by which we can remove this evil is to lengthen the reflectors, and consequently increase their breadth in the same proportion. Let us suppose, for example, that when the reflectors are five inches long, we can safely employ an apertureone-fourthof an inch in diameter; then if the plates are made ten inches long, we may use an aperturetwo-fourthsin diameter, the symmetry continuing as complete, and the light of the reflected images as intense, as when their lengthwas only five inches. By increasing the length of the reflectors, therefore, we increase also the quantity of light; but, unfortunately, this increase of length is unfavourable to the other properties of the instrument; for we must now use a lensL Lof a great focal length, which will render it necessary to receive the image at a great distance from the instrument.
In order to render the effect as brilliant as possible, the inclination of the reflectors should, in the present case, never exceed 30°, and might be even 36° or 45°. The objects should be selected as thin as possible, and none with dark tints ought to be admitted into the object-boxes.