The kaleidoscope

by David Brewster

In considering how this change might be effected, it occurred to me, that if M N, [Fig. 27], were a distant object, either opaque or transparent, it might be introduced into the picture by placing a lens L L, single or achromatic, at such a distance before the aperture A O B, that the image of the object may be distinctly formed in the air, or upon a plate of glass, the inner side of which was finely ground, and in contact with the ends A O, B O of the reflectors, the plane passing through A O B. By submitting this idea to experiment, I found it to answer my most sanguine expectations. The image formed by the lens at A O B became a new object, as it were, and was multiplied and arranged by successive reflexions in the very same manner as if the object M N had been reduced in the ratio of M L to L A, and placed close to the aperture.

Fig. 28.

The Compound or Telescopic Kaleidoscope is therefore fitted up as shown in [Fig. 28], with two tubes, A B, C D. The inner tube, A B, contains the reflectors as in [Fig. 27], and at the extremity C, of the outer tube C D, is placed a lens which, along with the tube, may be taken off or put on at pleasure. The focal length of this lens should always be much less than the length of the outer tube C D, and should in general be such that it is capable of forming an image at the end of the reflectors, when A B is pulled out as much as possible, and when the object is within three or four inches of the lens. When it is required to introduce into the picture very large objects placed near the lens, another lens of a less focal length should be used; and when the objects are distant, and not very large, a lens, whose principal focal length is nearly equal to the greatest distance of the lens from the reflectors, should be employed.

When this compound Kaleidoscope is used as a simple instrument for viewing objects held close to the aperture, the tube A B is pushed in as far as it will go, the cell with the object-plate is slipped upon the end C of the outer tube, and the instrument is used in the same way as the simple Kaleidoscope.

In applying the compound Kaleidoscope to distant objects, the cell is removed and the lens substituted in its place. The instrument is then directed to the objects, and the tube A B drawn out till the inverted images of the objects are seen perfectly distinct, or in focus, and the pattern consequently perfectly symmetrical. When this is done, the pattern is varied, both by turning the instrument round its axis, and by moving it in any direction over the object to which it is pointed.

When the object is about four inches from the lens, the tube requires to be pulled out as far as possible, and for greater distances it must be pushed in. The points suited to different distances can easily be determined by experiment, and marked on the inner tube, if it should be found convenient. In most of the instruments there is, near the middle of the tube A B, a mark which is nearly suited to all distances beyond three feet. The object-plates held in the hand, or the mirror-box placed upon a table, at a distance greater than five or six inches, may be also used when the lens L is in the tube. The furniture of a room, books and papers lying on a table, pictures on the wall, a blazing fire, the moving branches and foliage of trees and shrubs, bunches of flowers, horses and cattle in a park, carriages in motion, the currents of a river, waterfalls, moving insects, the sun shining through clouds or trees, and, in short, every object in nature may be introduced by the aid of the lens into the figures created by the instrument.

The patterns which are thus presented to the eye are essentially different from those exhibited by the simple Kaleidoscope. Here the objects are independent of the observer, and all their movements are represented with the most singular effect in the symmetrical picture, which is as much superior to what is given by the simple instrument, as the sight of living or moving objects is superior to an imperfect portrait of them. When the flame of a blazing fire is the object, the Kaleidoscope creates from it the most magical fireworks, in which the currents of flame which compose the picture can be turned into every possible direction.

In order to mark with accuracy the points on the tube A B, suited to different distances, the instrument should be directed to a straight line, inclined like m n, [Fig. 3], to the line bisecting the angular aperture A O B, and brought nearer to the centre O of the field. The perfect junction of the reflected images of the line at the points mʹ nʹ, &c., so as to form a star, or a polygon with salient and re-entering angles, will indicate with great nicety, that the tube has been pulled out the proper length for the given distance. In this way, a scale for different distances, and scales for different lenses, may be marked on the tube.

In the construction of the Tele-Kaleidoscope, as it may be called, the greatest care must be taken to have the lens of sufficient magnitude. If it is too small, the field of view will not coincide with the circular pattern, that is, the centre of the circular pattern will not coincide with the centre of the field; and this eccentricity will increase as the distance of the lens from the reflectors is increased, or as the object introduced into the picture approaches to the instrument. The boundary of the luminous field is also an irregular outline, consisting of disunited curves. These irregularities are easily explained. When the lens is too small, the luminous field is bounded by the brass rim in which the glass is fixed; and as this brass rim is at a distance from the reflectors, the portion of it presented to the angular aperture cannot be formed by successive reflexions into a continuous curve; and for the same reason, the upper sectors of the luminous field are larger than the lower ones, and consequently the centre of the pattern cannot coincide with that of the field. In order to avoid these defects, therefore, the diameter of the lens should be such, that when it is at its greatest distance from the reflectors, the field of view may be bounded by the arch A B, [Fig. 13], and not by the brass rim which holds the lens. This may be readily known by removing the eye-glass, and applying the eye at E when the lens is at its greatest distance. If the eye cannot see the brass rim, then the lens is sufficiently large; but if the brass rim is visible, the lens is too small, and must be enlarged till it ceases to become visible. Sometimes the lens has been made so small that the brass rim is seen not only at A B, but appears also above the angular point O, and produces a dark spot in the centre of the picture.