Page:The American Cyclopædia (1879) Volume X.djvu/349

 LENS LENT 343 the edges, it is called convexo-concave, or di- verging meniscus. Concave lenses are used in spectacles for the relief of near-sighted per- sons, and in the eyepiece of opera glasses and spy glasses of low power. Convex lenses are used for far-sighted persons and singly as mag- nifiers. They cause the rays of light which pass through them to converge toward the central line at right angles to their surfaces ; so that to an eye in the right position, rays from dif- ferent parts of an object make a greater angle than if they had not come through the lens. Convex lenses are also used in combination in telescopes and microscopes, in which the image formed by one lens is looked at under the magnifying power of a second. The image is formed by a convex lens, by means of its power to make the rays of light converge, which brings all the light that emanated from each point of the object again to a point in the air on the opposite side of the lens. These points of the image have nearly the same rela- tive position as the corresponding points in the object, and may be rendered visible by being received upon smoke or vapor, or as in the camera obscura and magic lantern upon a sheet. The image in the clear air can be seen by an eye placed in a line prolonged from the object through the image. If the image be formed by a single convex lens, it will on being mag- nified be found to have two principal imper- fections, arising from spherical and from chro- matic aberration. The nature of these imper- fections, and the means employed for overcom- ing them by the makers of optical instruments, are explained under ABERRATION, ACHROMA- TIC LENS, and APLANATIO LENS. The material employed in the construction of lenses for opti- cal instruments is generally crown glass which contains very little lead, and flint glass which contains much lead and has a greater refractive power. The glass should be perfectly homo- geneous and free from stria). The production of such glass in masses of sufficient size to make lenses for large telescopes is a work of great difficulty. The best specimens yet pro- duced have been from the manufactory of the Messrs. Chance of Birmingham, England, made by a process invented by Guinand, a Swiss optician, the details of which have never been made public. (See GLASS.) LENS, a town of France, in the department of Pas-de-Calais, on the Souchez, 9 m. N. N. E. of Arras; pop. in 1866, 5,738. Lace and woollens are manufactured here, and in the neighborhood there are coal mines. In August, 1648, the French under Cond6 obtained here a great victory over the Spaniards. LENT (Anglo-Sax, lencten, Ger. Lenz, Dutch lente, spring), the springtide fast of 40 days before Easter. In the Latin church it is called jejunium quadragesimale, "the fast of 40 days ;" and the first Sunday in Lent is called in the oldest Latin rituals Dominica in quadra- gesima, " the Sunday on the 40th day " (before Easter). Hence the almost identical appella- tions among Latin peoples : in Italy and Por- tugal quaresima, in Spain cuaresma, and in France careme (caresme). Roman Catholic theologians and many Protestants maintain that this fast is, in substance, of apostolic ori- gin ; such is the opinion of St. Jerome. But the greater number of Protestants consider it to be of ecclesiastical institution. The common opinion is that it was established as a prepara- tion for the great anniversaries of Christ's crucifixion and resurrection, and in remem- brance of his fast of 40 days in the wilderness. Some authors contend that in the beginning this preparatory fast was limited to the first four days of Holy Week, embracing a fast of 40 hours, which was gradually extended to 40 days; but according to others, the fast was one of 40 days from the very first. Be that as it may, the Latin term quadragesima and the Greek reaaapaKocr^ were applied before the 4th century to a period of 40 days before Easter set apart for fasting and prayer, begin- ning with what is now the first Sunday of Lent, and terminating on Holy Thursday. Within this period neither Greeks nor Latins at first fasted 40 days, the Sundays and Thurs- days being excepted by both, and also the Saturdays by the Greeks. As the general sen- timent declared in favor of fasting 40 days, the period was lengthened both in the East and West. At Rome it became 50 days, beginning with Quinquagesima week, and in the time of Pope Melchiades (311) it was extended to 60 days, beginning on Sexagesima Sunday. On the other hand, the Greeks began the fast on the 70th day from Easter, or Septuagesima Sunday. At length Gregory the Great (590) directed that the quadragesimal fast should be- gin on the 6th Sunday before Easter, and that all the intervening week days should be fasting days. As this, however, only gave 36 such days, the last four days of the preceding week were added either by that pope or by Gregory II. (715), the solemn fast thus beginning on Ash Wednesday, which thenceforward was called caput jejunii, " the beginning of the fast." There is also considerable uncertainty regard- ing the nature of the obligation of fasting. The fasts of Holy Week seem to have been kept by all as obligatory; but the others, it is thought, were assumed as voluntary. The general custom came at length to be a gene- ral law. The council of Laodicea (about 363) prescribed entire abstinence from food on Holy Thursday and the exclusive use of " dry food " during all the fast days of Lent. The council of Orleans in 541 commanded that those who did not keep Lent should be considered as trans- gressing the law of the church ; and the eighth council of Toledo in 646 forbade the use of flesh meat. Wine, oil, and animal food were prohibited on fasting days, and are so still in the Greek church. Their use in the Latin church was made one of the grounds of sepa- ration in the time of Photius. By degrees in the West the use of all kinds of food, except flesh,