rhomb (b silent), rhombe, rhombus

[Fr. Gr. eidos =form, and ichthys a fish.] rhombe, from Lat. rhombus ; Gr. rhombos a spin- ning-wheel, a rhombus, from rhembo to revolve, to totter; Sp. & Ital. rombo .] 1. Geom.: 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. 2. Crystall.: A rhombohedron (q. v.). Fresnel's rhomb: Rhomb. Optics : An apparatus for converting plane into circularly-polarized light [ Polarization of Light ]. It is a parallelopiped of gla , 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 polarizing angle of the gla ), and finally emerges at right angles from the opposite 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 circularly polarized. rhomb-porphyry, s. Petrol.: A porphyry which incloses large crystals of orthoclase, presenting a rhombic outline, result ing from a peculiar habit of twinning. First described from the vicinity of Christiania. rhomb-spar, s. [DOLOMITE.] rhomb- ( b silent), pref . [Rномво-.] rhomb-ovate, a . [RномBOID-OVATE.] Ichthyol.: A tropical genus of Pleuronectide (q. v.), but represented in the Mediterranean and on the coast of Japan. There are sixteen species, prettily_colored and ornamented with ocellated spots. In a few species, the adult males have some of the fin-rays prolonged into filaments. Rhomboid ichthys grand i squama , the Japanese form, ranging to the American coast, has the scales deciduous.