ON THE CONSTRUCTION AND USE OF THE TELESCOPIC
KALEIDOSCOPE, FOR VIEWING OBJECTS AT A DISTANCE.
We have already seen, in explaining the principles of the Kaleidoscope,
that a symmetrical picture cannot be formed from objects placed at
any distance from the instrument. If we take the simple Kaleidoscope,
and holding an object-box in contact with the reflectors, gradually
withdraw it to a distance, the picture, which is at first perfect in
every part, will, at the distance of one-tenth of an inch, begin to be
distorted at the centre, from the disunion of the reflected images;
the distortion will gradually extend itself to the circumference, and
at the distance of eighteen inches, or less, from the reflectors, all
the symmetry and beauty of the pattern will disappear. An inexperienced
eye may still admire the circular arrangement of the imperfect and
dissimilar images; but no person acquainted with the instrument could
endure the defects of the picture, even when the slightest distortion
only is visible at the centre.
As the power of the Kaleidoscope, therefore, in its simple form, is
limited to transparent objects, or to the outline of opaque objects
held close to the aperture of the reflectors, it becomes a matter of
consequence to extend its power by enabling it to produce perfectly
symmetrical patterns from opaque objects, from movable or immovable
objects at a distance, or from objects of such a magnitude that they
cannot be introduced before the opening of the reflectors. Without such
an extension of its power, the Kaleidoscope might only be regarded as
an instrument of amusement; but when it is made to embrace objects of
all magnitudes, and at all distances, it takes its place as a general
philosophical instrument, and becomes of the greatest use in the fine,
as well as the useful arts.
[Illustration: FIG. 27.]
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.
[Illustration: 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.
Instead of using two tubes, a lens is sometimes fitted into a tube
about an inch longer than the focal length of the glass, and this tube
is slipped upon the object end =A B O=, Fig. 21. This mode of applying
the lens is, however, inferior to the first method, as there is little
room for adjusting it to different distances; whereas with the long
tube all objects at a greater distance than four inches from the lens
may be introduced into the picture—a property which possesses very
peculiar advantages.
The extension of the instrument to distant objects is not the only
advantage which is derived from the use of the lens. As the position
for giving perfect symmetry is rather within the extremities of the
reflectors than without them; and as it is impossible to place movable
objects within the reflectors, we are compelled to admit a small
error, arising principally from the thickness of the objects, and from
the thickness of the plate of glass which is necessarily interposed
between the objects and the reflectors. The compound Kaleidoscope,
however, is entirely free from this defect. The image of a distant,
or even of a near object, can be formed within the reflectors, and in
the mathematical position of symmetry; while, at the same time, the
substitution of the image for the object itself, enables us to produce
all the changes in the picture which the motion of the object could
have effected, merely by turning the instrument round its axis, or by
moving it horizontally, or in any other direction across the object.
This instrument may be advantageously placed upon a stand like a
telescope, and may either have a partial motion of rotation by means
of a ball and socket, as shown in the figure, or what is better, a
complete motion of rotation round the axis of the tube =C D=, within a
brass ring, occupying the place of the ball and socket.[5]
[5] An instrument called _The Improved Kaleidoscope_ has been recently
brought out in Paris. It is merely the Telescopic Kaleidoscope
_deteriorated_. It consists of a lens _fixed_ at the distance of
about two inches in front of the reflectors, and can therefore give
symmetrical pictures of objects only at _one distance_, while it cannot
be used as an ordinary Kaleidoscope. The instrument described in the
preceding page, with a lens that can be slipped off, is a much better
Kaleidoscope.