CHAPTER IX.
ON THE ILLUMINATION OF TRANSPARENT OBJECTSIN 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.
Fig. 24.
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-glasscone. 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, whereM A N Cis a portion of the cone, fixed to the endE Fof the Kaleidoscope, andm n o pthe object-box. If the angle formed by the sides of the cone is such that a ray of the sun’s light falling uponM, the upper margin of the reflecting surface of the cone, is reflected too, the lower side of the object-box, then all the rays of a beam of thesun’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 onNis reflected tom, 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 glassm n,o pbeing fixed in a cylindrical case of glassm n o p, so that raysR R, either parallel or diverging, may be reflected from the coneA B C D, and after passing through the transparent cylindrical rimm n o p, illuminate the objects. Another coneA M N C, with its angle less than a right angle, may be joined toA B D C, so as to throw the rays obliquely, intom nando p, and also, if desired, upon the front glassm oof the object-box, the direct rays being excluded by an opaque diskS S. In this construction, the outer face of the glassm oshould 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 stopS 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. IfA B C D,Fig. 25, is the section of a polyhedral cone of flint or plate glass, a portionm n o pis cut out of its baseA B, to form an object-box for the reception of the pieces of coloured glass, or other objects. Thesidesm 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 apexD C Emay, in some cases, be cut off, and the polygonal sectionD Eblackened in order to prevent the introduction of direct light, and act as the stopS Sin the preceding figure.
Fig. 25.
Fig. 25.
A similar effect will be obtained from a polyhedral solid, of a hemispherical form, as shown inFig. 26, whereA 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 boxm n o p. The frontD E Cmay have an apex, as inFig. 25, or may be made spherical to act as a condensing lens, the surface of which may be blackened whennecessary, 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.
Fig. 26.
Fig. 26.
When the light is strong, a circular disk of fine grained white paper may be advantageously placed upon the outer facem oof the object-box.