The kaleidoscope

by David Brewster

CHAPTER VII.

ON THE CONSTRUCTION AND USE
OF THE SIMPLE KALEIDOSCOPE.

In order to construct the Kaleidoscope in its most simple form, we must procure two reflectors, about five, six, seven, or eight inches long. These reflectors may be either rectangular plates, or plates shaped like those represented in [Fig. 1], having their broadest ends A O, B O, from one to two inches wide, and their narrowest ends a E, b E, half an inch wide. If the reflectors are of glass, the newest plate glass should be used, as a great deal of light is lost by employing old plate glass, with scratches or imperfections upon its surface. The plate glass may be either quicksilvered or not, or its posterior surface may be ground, or covered with black wax, or black varnish, or anything else that removes its reflective power. This, however, is by no means absolutely necessary, for if the eye is properly placed, the reflexions from the posterior surface will scarcely affect the distinctness of the picture, unless in very intense lights. If it should be thought necessary to extinguish, as completely as possible, all extraneous light that may be thrown into the tube from the posterior surface of the glass plates, that surface should be coated with a varnish of the same refractive and dispersive power as the glass.

If the plates of glass have been skilfully cut with the diamond, so as to have their edges perfectly straight, and free from chips, two of the edges may be placed together, as in [Fig. 17 (p. 49)], or one edge of one plate may be placed against the surface of the other plate, as shown in the section of Mr. Bates’s Kaleidoscope. But if the edges are rough and uneven, one of them may be made quite straight, and freed from all imperfections, by grinding it upon a flat surface, with very fine emery, or with the powder scraped from a hone. When the two plates are laid together, so as to form a perfect junction, they are then to be placed in a brass or any other tube, so as to form an angle of 45°, 36°, 30°, or any even aliquot part of a circle. In order to do this with perfect accuracy, direct the tube containing the reflectors to any line, such as m n, [Fig. 2], placed very obliquely to one of the reflectors A O, and open or shut the plates till the figure of a star is formed, composed of 8, 10, or 12 sectors, or with 4, 5, or 6 points, corresponding to angles of 45°, 36°, and 30°. When all the points of the star are equally perfect, and none of the lines which form the salient and re-entering angles disunited, the reflectors must be fixed in that position by small arches of brass or wood A B, a b, [Fig. 21], filed down till they exactly fit the space between the open ends of the plates. The plates must then be kept in this position by pieces or wedges of cork or wood, or any other substance pushed between them and the tube. The greatest care, however, must be taken that these wedges press lightly upon the reflectors, for a very slight force is capable of bending and altering the figure even of very thick plates of glass.

When the reflectors are thus placed in the tube, as in [Fig. 21], their extremities a E, b E, next the eye, must reach to the very end of the tube, as it is of the greatest importance that the eye get as near as possible to the reflectors. The other ends of the reflectors A O, B O, must also extend to the other extremity of the tube, in order that they may be brought into contact with the objects which are to be applied to the instrument. In using transparent objects the cell or box which contains them may be screwed into the end of the tube, so as to reach the ends of the reflectors, if they happen to terminate within the tube; but an instrument thus constructed is incapable of being applied to opaque objects, or to transparent objects seen by reflected light.

Fig. 21.

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.