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LEFT RHODODENDRON 26 RHONDDA ers are large, varying in color from pale carmine to lilac. This species is quite hardy in Great Britain; as is also R. ponticum, a very similar species, with narrower and more pointed leaves, which are of the same color on both sides, a native of western Asia, and apparently also of the S. of Spain. R. catawbiense, a native of the S. parts of the Alle- ghenies, with large purple flowers; R. Caucasicum, the name of which indi- cates its origin; and R. arboreum, a native of Nepal, with very dense heads of large scarlet flowers. Most of the RHODODENDRON extremely numerous varieties now com- mon in our gardens and shrubberies have been produced from them by hy- bridizing or otherwise. Many splendid species of rhododen- drons were discovered in the Himalayas, the Khasia hills, and other mountainous parts of India, and have been introduced into cultivation in Europe. R. Falconeri is described as in foliage the most su- perb of all, the leaves being 18 or 19 inches long. It is a tree 30 to 50 feet high, with leaves only at the extremities of the branches. It grows in eastern Nepal at an altitude of 10,000 feet. R. argenteum has flowers 4% inches long, and equally broad, clustered, and very beautiful. R. Maddeni, R. Aucklandii, R. Edgeworthii, and others have white flowers. R. Dalhousise is remarkable as an epiphyte, growing on magnolias, lau- rels, and oaks. It is a^ slender shrub, bearing from three to six white lemon- scented bells, 4% inches long, at the end of each branch. R. Nuttali has fragrant white flowers, said to be larger than those of any other rhododendron. All these belong to the Himalayas. In more southern latitudes, as on the Neilgherry Hills and on the mountains of Ceylon, R. no bile prevails, a timber tree 50 to 70 feet high, every branch covered with a blaze of crimson flowers. R. Keysii and R. Thibaudiense, also natives of the N. of India, have flowers with nearly tubular corolla. R. ferrugineum and R. hirsutum are small species, natives of the Alps. They are called Alpenrose (Alpine rose) by the Germans. They have small carmine-colored flowers. The flora of the Himalayas contains a num- ber of similar small species. R. antho- pogon and R. setosum, dwarf shrubs with strongly scented leaves, clothe the mountains in eastern Nepal. R. nivale is the most alpine of woody plants, spreading its small woody branches close to the ground at an elevation of 1,700 feet in Sikkim. R. lapponicum, a pro- cumbent shrub, with small flowers, grows as far N. as human settlements have reached in Europe, Asia, and the United States. An oil obtained from the buds of R. ferrugineum and R. hirsutum is used by the inhabitants of the Alps, under the name Olio di Marmotta, as a remedy for pains in the joints, gout, and stone. R. chrysanthum, a low shrub, with golden yellow flowers, a native of Siberia, is also used in gout and rheu- matism. R. cinnabarinum, a Himalayan species, poisons goats which feed on it. But the flowers of R. arboreum are eaten in India, and Europeans make a palat- able jelly of them. RHOMB, or RHOMBUS, in geometry, an oblique parallelogram whose sides are all equal. The diagonals of a rhombus bisect each other at right angles. The area of a rhombus is equal to half the product of its diagonals. Fresnel's rhomb, in optics, is an ap- paratus for converting plane into circu- larly polarized light. It is a parallelo- piped of glass, of such length and angles that a ray of light entering one small end at right angles, twice suffers total reflection within the rhomb at an angle of about 54° (depending on the polariz- ing angle of the glass), and finally emerges at right angles from the oppo- site small end. When the beam of light is plane polarized, and the rhomb is so arranged that its reflecting faces are inclined at an angle of 45° to the plane of polarization, the beam emerges circu- larly polarized. RHONDDA, MARGARET HAIG MACKWORTH, VISCOUNTESS, a wo- man prominent in British enterprises, born in 1883. The daughter of G. A. Haig of Penithon, Radnorshire, she mar- ried in 1908 Sir Humphrey Mackworth.