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.