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.
[Illustration: 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.
[Illustration: 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._
[Illustration: FIG. 39.]
[Illustration: 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.
[Illustration: 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._
[Illustration: FIG. 42.]
[Illustration: 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.
[Illustration: FIG. 44.]