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LENA

1053

LENS

daytime. The name means ghost, and was given because of their coming forth at night and because of their eerie appearance. They vary in size from that of a fox down to a mouse. The typical lemurs have fox-like faces; the eyes are large and round, the body is covered with soft, wooly fur, the tail is bushy. There is, however, much variation about them; in some the tail is absent, in other species it is of considerable length. In color there is a wide variation. They live in troops in the forest, feed on dates, other fruits and on insects, and some attack birds. They obtain water from juicy fruits. The aye-aye is an unusual form of lemur. Though the creatures really are harmless, because of their strange cries, nocturnal habits and curious appearance much superstition has grown up about them.

Le'na, a river of eastern Siberia, rises amid the mountains on the northwestern shore of Lake Baikal, flows northeast to Yakutsk, where it is more than six miles wide, then north to the Arctic Ocean, into which it empties by several mouths, forming a delta 250 miles wide. The entire length of the river is 3,000 miles, and the area of its basin 750,000 square miles. The Lena is the principal artery of the trade of eastern Siberia, navigation being open annually from Yakutsk northward from May until October.

Lenormant (le-ndr'mdn'), Charles, a distinguished archaeologist, was born at Paris, June i, 1802. Early in life he began the study of law, but miring a visit to Italy he became much interested in the study of archaeology, and to this his life was henceforth devoted. In 1828 he accompanied Champollion to Egypt, and, after his return to France, held various positions, including that of adjunct-professor to Guizot. He was made professor of Egyptology in the College of France in 1848, and died at Athens, Nov. 24, 1859. His son Francois, born in 1837, whose death occurred in 1883, also attained considerable fame as an archaeologist.

Len'ox Library, The, an institution founded in New York City in 1870 by James Lenox. Mr. Lenox had inherited a large fortune, and during many years devoted his time to collecting rare books and manuscripts. These collections he deeded to trustees for the public benefit, and erected a building to contain them which cost nearly one half million dollars.

Lens, a combination of two refracting surfaces bounded on each side by the same medium. Generally the lens is a trans-

Earent piece of glass bounded on each side y air. Since the only surfaces which can be ground in lathes with accuracy are spherical surfaces, practically all lenses are made with spherical surfaces. That is, the shape of a lens may be considered as the

shape of a figure bounded by two spheres. The following figures show four of the principal types of lenses:

FIG.  I.A  CONVERGING LENS

The line joining the centers A and B of the two spheres is called the principal axis of the lens. Note that this axis is perpendicular to the surface of the lens at the point where it passes through the surface. See Focus.

Lenses are divided into two principal classes, viz., diverging and converging.

A diverging lens ip one such that if a plane wave-front is incident upon it, the emergent wave-front will be convex on its advancing side; while a converging lens is one such that if a plane wave-front is incident upon it, the emergent wave-front will be concave on its advancing side. Whatever the incident wave-surface, a diverging lens makes the emergent surface more convex, and a converging lens makes the emergent surface less convex.

Lenses are generally ground in such a way that this emergent wave-front is very nearly spherical. The center, F, of this spherical surface is called the principal focus of the lens. (See Fig. 5.) The distance from the principal focus to the center of the lens is roughly called the focal length. The power of a lens is defined as the reciprocal of the focal length. The manner

FIG.  3.A   DIVERGING  LENS

in which a lens produces an image will be clear from Figs. 6 and 7. Considering any

Fig. a.    A diverging lens with one plane surface