(See Eyeglasses.) are certainly the most valuable of all optical instruments, though there is not the same science and mechanical ingenuity displayed in the making of them as in the construction of microscopes and telescopes. A man, especially if accustomed to spend his time among books, would be much to be pitied, when his sight begins to fail, could he not, in a great measure, restore it by the aid of spectacles; but there are some men whose sight cannot be aided by the use either of convex or concave glasses. The following method adopted by one of those to aid his sight is certainly worthy of notice:

When about sixty years of age, this man had almost entirely lost his sight, seeing nothing but a kind of thick mist, with little black specks which appeared to float in the air. He knew not any of his friends, he could not even distinguish a man from a woman, nor could he walk in the streets without being led. Glasses were of no use to him; the best print, seen through the best spectacles, seemed to him like a daubed paper. Wearied with this melancholy state, he thought of the following expedient.

He procured some spectacles with very large rings; and, taking out the glasses, substituted in each circle a conic tube of black Spanish copper. Looking through the large end of the cone he could read the smallest print placed at its other extremity. These tubes were of different lengths, and the openings at the end were also of different sizes; the smaller the aperture the better could be distinguished the smallest letters; the larger the aperture the more words or lines it commanded; and consequently the less occasion was there for moving the head and the hand in reading. Sometimes he used one eye, sometimes the other, alternately relieving each, for the rays of the two eyes could not unite upon the same object when thus separated by two opaque tubes. The thinner these tubes, the less troublesome are they. They must be totally blackened within so as to prevent all shining, and they should be made to lengthen or contract, and enlarge or reduce the aperture at pleasure.

When he placed convex glasses in these tubes, the letters indeed appeared larger, but not so clear and distinct as through the empty tube: he also found the tubes more convenient when not fixed in the spectacle rings; for when they hung loosely they could be raised or lowered with the hand, and one or both might be used as occasion required. It is almost needless to add, that the material of the tubes is of no importance, and that they may be made of iron or tin as well as of copper, provided the inside of them be sufficiently blackened. See La Nouvelle Bigarure for February 1754, or Monthly Magazine for April 1799.