ON THE ILLUMINATION OF TRANSPARENT OBJECTS IN THE KALEIDOSCOPE.
When the Kaleidoscope is directed to the sky, or to a luminous object,
such as a gas flame, or the flame of a candle, a uniform tint is seen
through the pieces of glass, or other transparent fragments that
have flat surfaces, and there is a certain degree of hardness in
their outlines. When the instrument is not opposite the flame, but
directed to one side of it, the light enters the transparent fragments
obliquely, and a much finer effect is produced. The pattern, indeed,
changes very considerably by making the Kaleidoscope move round the
flame. An excellent effect is obtained, as we have already stated,
by directing the tube between two bright lights; and the richness of
the symmetrical pattern increases with the number of lights which
illuminate the objects. As it would be inconvenient to adopt such a
mode of illumination, it becomes of importance to have some contrivance
attached to the instrument, by which we can illuminate the objects by
light falling upon them in different directions.
[Illustration: FIG. 24.]
The simplest method of thus illuminating the objects, is to fix on the
end of the Kaleidoscope the portion of a metallic or silvered-glass
cone. The light of a bright flame, placed in front of the cone, will
be reflected from its interior surface, and fall obliquely on the
fragments of coloured glass. In many cases the effect will be increased
by placing in the base or mouth of the cone a circular stop, or opaque
disk, in order to prevent any light from falling directly upon the
objects, their oblique illumination being produced solely by the
rays reflected from the interior surface of the cone. This will be
understood from the annexed figure, where =M A N C= is a portion of
the cone, fixed to the end =E F= of the Kaleidoscope, and _m n o p_
the object-box. If the angle formed by the sides of the cone is such
that a ray of the sun’s light falling upon =M=, the upper margin of the
reflecting surface of the cone, is reflected to _o_, the lower side
of the object-box, then all the rays of a beam of the sun’s light
incident upon the upper half of the conical surface, will be reflected
upon the object-box; and, for the same reason, if a ray falling on
=N= is reflected to _m_, all the other rays falling on the lower half
of the conical surface will be reflected upon the object-box, and
illuminate obliquely the objects which it contains.
In the Kaleidoscopes of more recent construction, the object-box is
made transparent throughout,—the plates of glass _m n_, _o p_ being
fixed in a cylindrical case of glass _m n o p_, so that rays =R R=,
either parallel or diverging, may be reflected from the cone =A B C D=,
and after passing through the transparent cylindrical rim _m n o p_,
illuminate the objects. Another cone =A M N C=, with its angle less
than a right angle, may be joined to =A B D C=, so as to throw the rays
obliquely, into _m n_ and _o p_, and also, if desired, upon the front
glass _m o_ of the object-box, the direct rays being excluded by an
opaque disk =S S=. In this construction, the outer face of the glass
_m o_ should not be ground, as it would prevent the admission of the
light to the objects, the exclusion of external objects, the purpose
for which the grey glass is required, being effected by the stop =S
S=. The illuminating cone may be of tin, or, what is much better, of
plated copper, which reflects more light than any other metal, and it
must be so attached to the tube containing the reflectors, as to have a
rotatory motion.
The same kind of lateral illumination may be obtained from polyhedral
cones or hemispheres of solid glass. If =A B C D=, Fig. 25, is the
section of a polyhedral cone of flint or plate glass, a portion _m
n o p_ is cut out of its base =A B=, to form an object-box for the
reception of the pieces of coloured glass, or other objects. The sides
_m n_, _m o_, _o p_, being highly polished, rays of light, either
parallel, as emanating from the sun, or diverging from artificial
sources of light, will be refracted and fall obliquely upon the faces
of the object-box, and illuminate its contents with the irregular
prismatic spectra which are formed by refraction. The apex =D C E= may,
in some cases, be cut off, and the polygonal section =D E= blackened in
order to prevent the introduction of direct light, and act as the stop
=S S= in the preceding figure.
[Illustration: FIG. 25.]
A similar effect will be obtained from a polyhedral solid, of a
hemispherical form, as shown in Fig. 26, where =A C=, =C D=, =D E=, =B
F=, =F G=, =G E=, are polished facets, by which parallel or diverging
rays immediately before it, or incident in any lateral direction, may
be refracted so as to illuminate by the prismatic rays the objects in
the box _m n o p_. The front =D E C= may have an apex, as in Fig. 25,
or may be made spherical to act as a condensing lens, the surface of
which may be blackened when necessary, for the purpose of excluding
the direct light. These illuminators may be attached in various ways
to the tube containing the reflectors, so to have a rotatory motion in
front of them.
[Illustration: FIG. 26.]
When the light is strong, a circular disk of fine grained white
paper may be advantageously placed upon the outer face _m o_ of the
object-box.