In the preceding applications of the Kaleidoscope, the great difficulty to be overcome arises from the smallness of the aperture which can be obtained at the eye-end of the reflectors. If we take a larger aperture, for the purpose of gaining more light, the light of the reflected images is diminished by this very circumstance, and the picture loses its symmetry at the centre. The only method by which we can remove this evil is to lengthen the reflectors, and consequently increase their breadth in the same proportion. Let us suppose, for example, that when the reflectors are five inches long, we can safely employ an aperture _one-fourth_ of an inch in diameter; then if the plates are made ten inches long, we may use an aperture _two-fourths_ in diameter, the symmetry continuing as complete, and the light of the reflected images as intense, as when their length was only five inches. By increasing the length of the reflectors, therefore, we increase also the quantity of light; but, unfortunately, this increase of length is unfavourable to the other properties of the instrument; for we must now use a lens =L L= of a great focal length, which will render it necessary to receive the image at a great distance from the instrument. In order to render the effect as brilliant as possible, the inclination of the reflectors should, in the present case, never exceed 30°, and might be even 36° or 45°. The objects should be selected as thin as possible, and none with dark tints ought to be admitted into the object-boxes. CHAPTER XVI. ON THE CONSTRUCTION OF KALEIDOSCOPES WHICH COMBINE THE COLOURS AND FORMS PRODUCED BY POLARIZED LIGHT.