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LENT

1054

LEO III

luminous point, O, we may determine its image I by graphical means, if we remember the two following fundamental facts: i. A ray falling on a lens in a direction parallel to its axis passes on emergence through its principal focus. 2. An incident ray which passes through the principal focus of a lens will emerge in a direction parallel to the axis of the lens. Thus the rays leaving O in Fig. 6 and passing through F and F', intersect at I and thus determine I as the image of O. A similar process applied to every other point on the arrow O will give the corresponding points on the image-arrow I. In general, the outer portions of a lens behave rather differently from the portions near the center. Hence a single lens, to

Fig. 4.    A converging lens with one plane surface

Fig. 5.—Focal length oZ a converging lens

give good definition, must be used with small aperture; that is, the pencil of rays which can be successfully used with a single lens is small. But if a lens is limited

Fig. 6.—Image of a body produced by a converging lens

by a diaphragm, so that it admits only a small pencil of rays, the image will not be bright. In order to overcome this difficulty the so-called achromatic lenses have been invented. The discussion of these lenses is a subject top advanced for this place; but an achromatic lens may be simply denned as one which will do for its whole

Fig. 7.—Image of a body produced by a diverging lens

aperture what the single lens does only for its center. The variety of lenses used in practice is enormous, almost each different purpose requiring a different lens. The best treatment of lenses extant is to be

SECTION OP A LENTICEL

found in Winkelmann's Handbook of Physics. It is written by Czapski, and has not been translated into English.

Lent, the 40 days' fast before Easter Sunday, instituted in the early history of the Christian church as a preparation for the anniversary of Christ's resurrection and also as a memorial of his 40 days' fast in the wilderness. The rigor of the ancient observance, which excluded all flesh and even the so-called white meats, has been much relaxed, but the principle of permitting but. one meal, with a slight refection or collation, has been retained by all churches that recognize the obligation of keeping Lent. In the Church of England and the Protestant Episcopal Church of the U. S. Lent is observed with special services and with proper collects and prayer; but the fast is left to the conscience and discretion of each individual.

Len'ticel. In stems in which bark is produced, the cork-cells as the point correspond ing to the stomata of the epidermis become rounded and loosened from one another. Under this strain the epidermis ruptures-at the stoma, and a powdery mass of cells is exposed through a slit-like opening, the whole structure being called a lenticel. These lenticels are commonly seen on young bark, and serve to place the living cells within in connection with the outside air.

Leo, the name of 13 popes of the Roman Catholic church, of whom Leo I, Leo III, Leo X and Leo XIII are most worthy of mention.

Leo I, surnamed The Great, was born of a distinguished family at Rome, about the close of the 4th century; and on the death of Sixtus III in 440 Leo was chosen his successor.* It is in his pontificate that the regular series of pontifical letters and decretals may be said to have commenced. They exhibit remarkable activity and zeal, and are often quoted as evidence of the extent of pontifical jurisdiction at that time. In a council held at Rome in 449 Leo set aside the proceedings of the "Robber Synod" of Ephesus, which had decided in favor of Eutyches, and summoned a council at Chalcedon, in which his celebrated letter was accepted "as the voice of Peter." Leo died in 461.

Leo III was born at Rome in 750, and succeeded Hadrian I in 795. His pontificate was far from being a peaceful one. In 800 Charlemagne came to Rome, where he was crowned and saluted as emperor by Leo, and the temporal sovereignty of the pope over the Roman city and state was formally established, Leo became involved in a