ON THE APPLICATION OF THE KALEIDOSCOPE TO THE MAGIC
LANTERN, SOLAR MICROSCOPE, AND CAMERA OBSCURA.
[Illustration: FIG. 50.]
In the various forms of the Kaleidoscope which have been described
in the preceding chapters, the pictures which it creates are visible
only to one person at a time; but it is by no means difficult to fit
it up in such a manner as to exhibit them upon a wall to any number of
spectators. The necessary limitation of the aperture at the eye-end
of the instrument, however, is hostile to this species of exhibition,
as it requires a very intense light for the purpose of illuminating
the objects. The general principle of the apparatus requisite for this
purpose is shown in Fig. 50, where =C D G F= is the tube containing
the reflectors =A O E=, etc. The objects from which the pictures are
to be created are placed in the cell =C D=, which may be made either
to have a rotatory movement round the axis of the tube, or to slide
through a groove, like the sliders of a magic lantern. These objects
are powerfully illuminated by a lens =B=, which concentrates upon them
the direct light of the lamp or candle =H=, and also the part of the
light which is reflected from the mirror =M N=. At the eye-end =E=
of the Kaleidoscope, is placed a lens =L L=, close to the end of the
reflectors, and having its centre coincident with the centre of the
aperture at =E=. In order that this lens may form behind it an image =P
P= of the objects placed in the object-plate =C D=, its focal length
must be less than the length =A E= of the plates. If the focal length
of =L L= is so small as one-half of =A E=, then it follows, from the
principles of optics, that the distance =L P= at which the image is
formed behind the lens, will be precisely equal to the distance =A E=
of the object; but this is obviously too small a distance, for the
diameter of =P P= would be equal only to the apparent diameter of the
circular aperture of the Kaleidoscope, or to twice =A O=. Hence it is
necessary, that the focal length of the lens =L L= be less than =A E=,
and greater than half of =A E=. Two-thirds, or three-fourths of =A E=
will be found to be a suitable focal length; for if it is larger than
this, the image will be formed upon the wall or screen at too great a
distance from the instrument.
When the instrument is thus fitted up, an enlarged image of the pattern
will be thrown upon the wall, which must be covered with white paper,
or some white ground, in order to exhibit the colours to advantage. By
turning the object-plate round its centre, or, if it is a rectilineal
one, by pushing it through the groove, and at the same time giving it
a rotatory motion, the pattern on the wall will undergo every possible
transformation, and exhibit to the spectators, in a magnified form,
all those variations which have been observed by applying the eye to
the Kaleidoscope.
When the preceding apparatus is used in daylight, so that the
objects are illuminated by the rays of the sun, the mirror =M N= is
unnecessary. The Kaleidoscope, etc., must, however, be attached to the
part of the frame of a Solar Microscope, which goes into the aperture
in the window-shutter.
As the most brilliant light is obtained from the burning of oxygen,
either by itself, or along with coal gas, as in the Bude light,[8] a
lamp of this kind is peculiarly fitted for displaying the pictures
of the Kaleidoscope to a number of spectators. One of Mr. Bate’s
Polycentral Kaleidoscopes was, many years ago, fitted up with a lamp
of this kind, for exhibition, at the lectures on natural philosophy,
delivered at Guy’s Hospital, by that eminent chemist, the late William
Allen, Esq., F.R.S.
The patterns which are created by the Telescopic Kaleidoscope from
natural objects, or from objects independent of the instrument, may, in
like manner, be exhibited to several spectators at once. If the objects
are in a room, such as bunches of flowers, statues, human figures, or
large pictures, they must be placed in one apartment, and strongly
illuminated. The lens must then be placed upon the end =A O= of the
Kaleidoscope, the object-plate =C D= having been removed, and must be
so adjusted that the images of the objects may fall exactly upon the
end =A O= of the reflectors. The objects may be placed at any distance
from the lens, from six inches to twelve feet, according to their
magnitude, and the pictures will be exhibited with great distinctness
and effect upon the wall of the other apartment. If a blazing fire is
employed, the most brilliant display of fireworks may be exhibited.
When the objects are out of doors, such as trees, shrubs, etc., the
Kaleidoscope, with its two lenses, must be fixed in the circular
opening of a window-shutter, and the picture received upon white paper,
or any other suitable ground, as in the Camera Obscura.
[8] This light was first proposed by myself. See _Edinburgh Review_,
April 1833, vol. lvii. p. 192.
Similar effects may be produced in a portable Camera Obscura, by
placing the apparatus =C F L L G D= in the moveable drawer of that
instrument. If the lens =L L= is of such a focal length as to admit
the formation of the image within the instrument, the picture will
be finely displayed upon the ground glass, and may be copied with
considerable exactness.
In the preceding applications of the Kaleidoscope, the great difficulty
to be overcome arises from the smallness of the aperture which can
be obtained at the eye-end of the reflectors. If we take a larger
aperture, for the purpose of gaining more light, the light of the
reflected images is diminished by this very circumstance, and the
picture loses its symmetry at the centre. The only method by which we
can remove this evil is to lengthen the reflectors, and consequently
increase their breadth in the same proportion. Let us suppose, for
example, that when the reflectors are five inches long, we can safely
employ an aperture _one-fourth_ of an inch in diameter; then if the
plates are made ten inches long, we may use an aperture _two-fourths_
in diameter, the symmetry continuing as complete, and the light of
the reflected images as intense, as when their length was only five
inches. By increasing the length of the reflectors, therefore, we
increase also the quantity of light; but, unfortunately, this increase
of length is unfavourable to the other properties of the instrument;
for we must now use a lens =L L= of a great focal length, which will
render it necessary to receive the image at a great distance from the
instrument.
In order to render the effect as brilliant as possible, the inclination
of the reflectors should, in the present case, never exceed 30°, and
might be even 36° or 45°. The objects should be selected as thin as
possible, and none with dark tints ought to be admitted into the
object-boxes.