The origin of these valuable instruments is uncertain: that the ancients were acquainted with the laws of refraction is beyond all doubt, since they made use of glass globes filled with water to produce combustion; and in Seneca we find the following very curious passage--"Litterae, quamvis minutiae et obscurae, per vitream pilam aqua plenam majores clarioresque cernuntur;" yet thirteen centuries elapsed ere spectacles were known. It is supposed that they were first invented by _Salvino_ or _Salvinio Armati_; but he kept his discovery secret, until Alessandro de Spina, a monk in Pisa, brought them into use in 1313. Salvino was considered their inventor, from the epitaph on his tomb in the cathedral church in Florence: "Qui giace Salvino d'Armato, degl' Armati di Firenze, inventor delli occhiali, &c., 1317." Another circumstance seems to add weight to this presumption: _Luigi Sigoli_, a contemporary artist, in a painting of the Circumcision, represents the high-priest Simeon with a pair of spectacles, which, from his advanced age, it was supposed he might have needed on the occasion. LEECHES. The origin of their introduction in the practice of medicine is uncertain. They were well known to the ancients under the name of _hirudo_. Thus Horace: Non missura cutem nisi plena cruoris hirudo. The Greeks called them [Greek: Boella], and Pliny states that elephants were often cruelly tormented by them when they swallowed any of these worms in their water: "Cruciatum in potu maximum sentiunt hausta hirudine, quam sanguisugam vulgo coepisse appellari adverto." Leeches are oviparous, and their ova are discharged in one involucre near the surface and margin of pools, and are hatched by the heat of the sun. They do not cast their skin, as is generally supposed, but merely throw off a tough slimy membrane, which appears to be produced by disease, and from which they get disencumbered by straining themselves through grass and rushes. During winter they remain in a torpid state. They are most tenacious of life; some say they can live for several days in the exhausted receiver of the air-pump, and in other media destructive of other animals. This phenomenon is attributed to the slow oxygenation of the blood in the respiratory vesicles. In regard to their food we are ignorant, although Dr. Johnson says that they live by sucking the blood of fish and reptiles. The collection of leeches constitutes a lucrative trade on the Continent, but more particularly in France, where it is called a leech-fishery, and where, in Paris alone, three millions are annually consumed. The following is an interesting description of the miserable people engaged in this occupation from the _Gazette des Hopitaux_.