convex pebbles, are the same as described for concave or convex glasses. This consideration should weigh with those who are apt to be misled by the pretensions of the unprincipled; for pebbles have, in common with many other crystals, a double refracting property, which, if the pebble is cut carelessly, exhibits itself by painfully affecting the vision; two objects, instead of one, are seen, causing a confused and agitating sense of indistinctness, which, in proportion to the exertion of the eye to overcome it, is the more tiresome and distressing. Such faulty and blemished articles, technically called _wasters_, are refused by the optician of any real respectability and character, but are eagerly bought up by those venders, whose object is to purchase what costs them the least money, alike ignorant of, and indifferent to, any other consideration. Pebbles, therefore, like all the precious stones, which, in fact, are crystals, cannot be properly cut or sliced except in a parallel direction to their natural faces. Periscopic or meniscus lenses were introduced in the year 1770, and revived at a later period by Dr. Wollaston. Their properties were investigated by many scientific persons, and again candidly examined by Mr. W. Jones; yet, notwithstanding the oblivion to which they were consigned by the universal consent of practical opticians, they are still foisted on the public, under various disguises, and embellished with sundry new appellations. They are concave on one side, and convex on the other, the inner and outer curve differing, in order to produce a focus. When placed in the spectacle-frame, their convex surface is always from the eye; this necessarily exposes their centres to be rubbed and scratched. The halo, or aberration of light, is greater in lenses of this form than in any other. This can be easily proved by placing a double convex, and a periscopic lens, each of the same diameter and focus, say one-and-a-half inch diameter, and three or four inch focus, edgeways together. Let the image of a lighted candle be thrown against a white wall or paper, in a dark situation, through the two lenses, and it will be conspicuously seen that the periscopic lens exhibits this dazzling indistinctness, while in the double convex lens it is scarcely perceptible. Parallel or flat surface lenses are produced from tools perfectly level, and without any curvature. For spectacle purposes their only use is as shades or shields for the eye, to protect it from the effects of bright and dazzling objects, dust, and wind. They are, however, indispensable in the construction of sextants, artificial horizons, and other mathematical instruments.