CHAPTER IX.

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.

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 м, 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 cyclindrical 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.

If

The same kind of lateral illumination may be obtained from polyhedral cones or hemispheres of solid glass. 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.

A similar effect will be obtained from a polyhedral solid, of a hemispherical form, as shown in Fig. 26, where A C, C D, DE, B F, F G, GE, 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 mnop. 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 illumina

tors may be attached in various ways to the tube containing the reflectors, so to have a rotatory motion in front of them.

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.