The kaleidoscope

by David Brewster

CHAPTER XII.

ON THE CONSTRUCTION AND USE OF ANNULAR
AND PARALLEL KALEIDOSCOPES.

In the instruments already described, the pictures which they create, though they may be made of various outlines, have all a centre to which the reflected images are symmetrically related. The same instruments give an annular pattern, or a pattern returning into itself, and included between two concentric circles, by keeping the objects from the central part of the aperture; but as such a pattern can never have its greatest radius more than the breadth of the mirror, and as annular patterns of a very great radius, where the eye can see only a portion of them at a time, are often required, it becomes of importance to adapt the Kaleidoscope for this species of ornament.

Fig. 37.

Let A C B D, [Fig. 37], be two plane mirrors, and let their inclination be measured by the angle A O B; then, if the eye is placed between C and D, it will observe the reflected images of the objects which are placed before the aperture A C B D, arranged, in the annular segment M A B N, round O, as a centre. The effect is exactly the same as if the reflectors had been continued to O, with this difference only, that the annular segment can never be complete. This defect in the segment arises from two causes: When the centre O is near C D, the defect is occasioned by the want of a reflecting surface to complete the ring, and not from any want of light in the reflected images; but when the centre O is remote from C D, the defect arises from the want of light in the last reflexions, as well as from the want of a reflecting surface.

The theory of the Annular Kaleidoscope is exactly the same as that of the common instrument; and therefore all the contrivances for producing symmetrical pictures, from near and distant objects, are applicable to this instrument. As the picture, however, never can return into itself, it is of no importance that the angle A O B be the aliquot part of a circle, the picture being equally complete at all angles. In order to have the most perfect symmetry with this Kaleidoscope, the eye should be placed at E, between the nearest ends of the reflectors, as it will there be nearer the plane of both reflectors than in any other position. If the two mirrors are brought nearer each other, so that their surfaces always pass through the point O, the deviation from perfect symmetry will diminish as the eye becomes more and more in the plane of both; and for the same reason the light of the field will be more brilliant.

When the point O is infinitely distant, the two reflectors become parallel to each other, as in [Fig. 38], and the series of reflected images extends in a straight line, forming beautiful rectilineal patterns for borders, &c. In this position of the reflectors the eye should be placed in the centre at E, and the symmetry of the picture and the light of the field will increase as the distance of the reflectors diminishes, or as their length is increased.

Fig. 38.

Two different kinds of instruments have been constructed on the preceding principles, the one by Mr. Dollond, and the other by Mr. John Ruthven, both of which possess very valuable properties.

Mr. Dollond’s Universal Kaleidoscope.

Fig. 39.

Fig. 40.

The instrument constructed by Mr. Dollond is represented in [Figs. 39], [40], and [41], in section, and is intended to unite the properties of a common Kaleidoscope, in which the reflectors are inclined at an angle of 30°, and also those of an Annular and a Parallel Kaleidoscope. [Fig. 39] represents the reflectors, etc., when they act as a common Kaleidoscope; and [Fig. 40] shows them when they form a parallel Kaleidoscope, an annular Kaleidoscope being formed when they have an intermediate position. The tube of the instrument is shown, in section, by T T; and to this tube is fixed, by the screws s s, a frame of metal, a b, to which the reflectors are fastened. The reflectors, which are made of the finest speculum metal, are shown at A O, B O, and are attached to plates of brass, c d, c d, whose breadth exceeds that of the reflectors so as to allow their extremities to descend below the point O, [Fig. 39]. A double spring, y x x y, is placed in the tube, so as to press upon the back of the reflectors, and keep them in contact, as shown in [Fig. 39], and is sufficiently elastic as to allow them to open, as in [Fig. 40]. The milled head M N, which passes through the lower part of the tube, carries, at its lower end, a very eccentric button or wheel, the least diameter of which is seen at m, [Fig. 39], and the greatest at m, [Fig. 40]. In the first position it has allowed the reflectors to come into contact at O. In the other position it has forced them open into the position of parallelism. By turning the milled head, the lower ends (O O) of the reflectors may be brought to any distance less than O O, so as to form an annular Kaleidoscope. The eye-end of the instrument is shown in [Fig. 41]. The lens E is placed in a slider, C D, which is to be moved according to the position of the reflectors, being a little above O, in [Fig. 39], opposite the centre of the tube in [Fig. 40], and at an intermediate position in the intermediate position of the reflectors.

Fig. 41.

This instrument is attached to a stand with a draw-tube, which screws into the bottom of a mahogany box. The object-plates, and the lens for introducing distant objects, are placed at the end of the instrument, in the same manner as those of the usual construction. Particular kinds of objects are selected for giving rectilineal borders.

Ruthven’s Universal Kaleidoscope.

Fig. 42.

Fig. 43.

The instrument constructed by Mr. Ruthven is also a Universal Kaleidoscope, which unites the properties of a Polyangular one with those of Annular and Parallel Kaleidoscopes. Its construction will be understood from [Figs. 42], [43], and [44], where A B E F G H represents a frame of iron or brass, which slips into the tube. The two sides A B, F H, of this frame, are kept together by four cross pieces, a b, c d, etc., the other two corresponding to these being invisible in the figure. The two reflectors, the ends of which are seen at Aʹ O, Bʹ O, are each fixed to a plate of metal p p, a section of which is seen in [Fig. 44]. Each plate of metal has four cylindrical pins, p, p, etc., both on its upper and under edge. The two pins nearest the ends of the reflectors pass through openings in the cross pieces a b, c d. On the top of the frame is placed a plate of brass M N Q P, in which are cut grooves e f, g h, k l, m n; e f and k l, and also g h and m n, being parallel to each other. This plate can be moved forwards and backwards between the cross pieces a b, c d, by means of a small screw S S, working in a female screw fixed upon the edge N O of the plate, and as the middle pins p p, attached to the plates which carry the reflectors, pass through the grooves, any change in the position of the plate M Q, produces a change in the distance, p p, of the pins, and consequently in the distance of the upper edges of the reflectors. By turning the screw S S, therefore, the upper edges of the reflectors may be either brought into contact, or separated to a distance regulated by the inclination of the grooves e f g h. A similar plate with a similar screw is placed upon the lower edges of the reflectors, so that we are furnished with the means of giving the plates any inclination to each other, or placing them at any distance within certain limits. For example, if the lower edges of the plates are in contact, we can vary the angle of their inclination by separating or closing their upper edges by means of the upper screw. By separating the lower edges, we give them the position for annular patterns, and by making the distance of the lower and upper edges the same, we obtain from them rectilineal patterns; and the figures of these annular and rectilineal patterns may be either contracted or expanded, by altering the distance of the plates when in this parallel position.

Fig. 44.