Since the aberration from symmetry increases, as the length of the reflectors is diminished, and since the light of the field diminishes from the same cause, it becomes extremely difficult to obtain correct figures, and uniformity of light in small instruments. In order to overcome these difficulties, as far as possible, the reflectors should be metallic, and may be either made of polished steel or polished speculum metal. The inclination at which they are fixed should not be less than 36° or 45°; and the eye-hole, which should not exceed ¹/₁₅th of an inch in diameter, must be placed as near as possible to the angular point. Since the aberration from symmetry increases with the distance of the object from the reflectors, and is much augmented in small instruments, the greatest care must be taken to have the objects at the least possible distance from the reflectors. To accomplish this, the objects themselves should be as thin and slender as they can be made; the colours should be brilliant and not gloomy; and they should be separated from the reflectors by a thin film of the most transparent mica, which is superior to glass of equal thickness, even if it could be got, from its extreme toughness and elasticity. The mica, indeed, is easily scratched, but if this should take place to any extent, it can easily be replaced by a new film. It would even be of consequence to bend the mica into a slight concavity, so as to permit the objects to lie rather within than without the extremity of the reflectors. In order to see the pattern with perfect distinctness, a lens must be placed at the end of the instrument; the focal length of this lens, however, must not be exactly equal to the distance of the objects from the eye, but as much greater as possible, so that the eye, by a little exertion, may be able to obtain distinct vision. The reason of this will be understood, by considering that the images of the objects, seen by reflexions, are thrown to a greater distance, as it were, from the eye, and could not therefore be seen distinctly by using a lens adjusted exactly to the nearest part of the picture. Consequently, the focal length of the lens must be a mean between the distances of different parts of the picture, that is, a little greater than that which is suited to the sector seen by direct vision. When small Kaleidoscopes are made with four or with three mirrors, the preceding directions are equally applicable, the greatest care being taken that the reflectors taper nearly to a point at the eye-end, so as not to leave an aperture greater than ⅟₁₅th of an inch in diameter. When they are made of solid glass, the focal length of the lens must be determined from the principles contained in the last chapter. The preceding instruments may be fitted up with a draw-tube and lens, and when it is required to introduce drawings of pictures and statues, small microscopic photographs may be employed. CHAPTER XIX.