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 a lens 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 in Chapter XIII. CHAPTER XV. ON THE APPLICATION OF THE KALEIDOSCOPE TO THE MAGIC LANTERN, SOLAR MICROSCOPE, AND CAMERA OBSCURA. [Illustration: FIG. 50.]