CHAPTER XIV.
ON KALEIDOSCOPES IN WHICH THE EFFECT ISPRODUCED BY TOTAL REFLEXION FROM THEINTERIOR SURFACES OF TRANSPARENT SOLIDS.
When light is incident upon the most perfectly polished metals, a very considerable quantity of it is absorbed, and even when the reflexion is made at the greatest obliquities, there is a very manifest difference in the intensity of the direct and the reflected pencil. In the total reflexion of light from the second surfaces of transparent bodies, the loss of light is very inconsiderable, and the reflexion is made with a degree of brilliancy far surpassing that of the most resplendent metals.
Fig. 49.
Fig. 49.
In constructing a Kaleidoscope upon this principle, we must procure a piece of glass entirely free from veins, and cut it into the form shown inFig. 49.[7]The two surfacesB O E,A O E, must beinclined at an angle which is the even aliquot part of a circle. They must be ground perfectly flat and highly polished, and the junctionO Emust be made as fine as possible. The upper surfaceA B Eshould be rough-ground, and the sideA B O, and the side atE, should be parallel and well polished. If the glass is colourless and good, the eye, when placed atE, will see the very same appearance as in the simple Kaleidoscope; and objects placed atA B Owill be arranged into the same beautiful figures. The only defects attending this form of the Kaleidoscope, are the loss of light occasioned by its passing through a mass of solid glass, not perfectly transparent, and the difficulty of obtaining a perfect junction of the two reflecting planes. The first of these evils is, however, counterbalanced by the great intensity of the light which suffers total reflexion; and the second does not exist when the Kaleidoscope is intended to give rectilineal or annular patterns.
In the construction of instruments of this kind, it is necessary to make the prism of glass longer than the distance at which the eye can see objects with perfect distinctness; that is, if the eye is capable of seeing objects distinctly at the distance of five inches, it will not perceive the same objects distinctly when they are placed at the end of a prism of glass five inches long. This singular effect arises from a property of plain lenses or pieces of plain glass, in consequence of which, they cause divergent rays to diverge from a point nearer the lens or plate, than that from which they radiated. It will therefore be more convenient, for many reasons, to make the glass prism only two or three inches long, and obtain distinct vision by means of alens placed at the eye-end of it; but, for the reason already mentioned, the focal length of the lens must be less than the length of the glass prism. The lens may even be joined to the prism, by grinding the eye-end into a spherical form, but the degree of convexity must be calculated upon the principles already stated.
The solid form of the Kaleidoscope is peculiarly fitted for polycentral instruments, as we have only to polish the side, which would otherwise have been left rough, the prism being supposed to be cut to the angles which are necessary to give symmetrical forms, according to the principles stated inChapter XIII.