When the preceding apparatus is used in daylight, so that the objects are illuminated by the rays of the sun, the mirror M N is 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 end A O of the Kaleidoscope, the object-plate C D having been removed, and must be so adjusted that the images of the objects may fall exactly upon the end A O of the reflectors. The objects may be placed at any distance from the lens, from six inches to twelve feet, according to 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 apparatus C F L L G D in the moveable drawer of that instrument. If the lens L L is 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 aperture one-fourth of an inch in diameter; then if the plates are made ten inches long, we may use an aperture two-fourths in diameter, the symmetry continuing as complete, and the light of the reflected images as intense, as when their length was 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 lens L L of 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.
CHAPTER XVI.
ON THE CONSTRUCTION OF KALEIDOSCOPES
WHICH COMBINE THE COLOURS AND FORMS
PRODUCED BY POLARIZED LIGHT.
In the preceding chapters we have supposed that the objects are illuminated by common light, and that the forms which compose the symmetrical figure are those of material bodies. If we employ polarized light we may introduce into the Kaleidoscopic figures, the splendid colours produced by crystallized bodies, and also the forms which these colours assume, round the optical axes of crystals, or through different thicknesses of the doubly-refracting substance. The part of the polarizing apparatus which polarizes the light, may be a large Nicol’s prism, or a bundle of thin glass plates, or a single plate of black glass, fixed at the object end of the Kaleidoscope. The analysing part of the apparatus may be a Nicol’s prism, or plates of the sulphate of iodo-quinine, discovered by Dr. William Herapath, of Bristol. Owing to the thickness of a Nicol’s prism, it is not well fitted for the analyser, as it prevents the eye of the observer from getting sufficiently near the small eye-hole of the Kaleidoscope. The plates of the sulphate of the iodo-quinine, are therefore peculiarly adapted for analysers; and when they can be obtained of the same size as the angular aperture of the Kaleidoscope, with fixed reflectors, or of the whole circular aperture when the reflectors are movable, they will also form the best polarizers.