HELMET. 744 HELMHOLTZ. through which the warrior could see and breathe. It was so lieavy that it was put on only at the moment of combat, and only when the knight fouglit on liorseback. It was a very effective defense against a cut from a mediaeval weapon, but no protection against bruises. In great contrast with this was the headpiece worn by the Saracens, with whom the Crusaders came into contact. They used light globular helmets of iron, richly ornamented and adorued with plumes. In the fourteenth century the heavy helmet was to a great e.xtent discarded by the Western knights for the lighter bascinet; but it continued to be used in tournaments until the eighteenth century. As the employment of firearms became more general, helmets naturally lost their utility, especially as regards the face. Those still remaining are, in the equipment of troops, limited for the most part to heavy cavalry, afl'ord no protection to the face, and must be considered as rather for ornament than use. Firemen wear a heavy headpiece of leather and brass to protect them as far as possible from falling brands and cinders at conflagrations. In India and other hot climates, helmets of white felt, with the additional screen of rolls of linen, are worn by the military as well as by members of the civil service to protect them from the hot rays of the sun. In heraldry, the helmet, from the early simple form known as the Norman, came at a later period to vary in shape according to the de- gree of the person who bore it, and helmets were set over coats of arms to bear the crest and to indicate by their form the rank of the bearer. The following forms of helmet are in use in English heraldry; (1) The helmet assigned to the King and to the princes of the blood royal ; it is full-faced, composed of gold, and has the visor divided by si.x projecting bars. (2) The helmet of a duke or marquis is of steel, with five bars of gold. ( 3 ) The helmet of earls, viscounts, and barons is of silver, adorned with gold, has five bars, and is set in profile. (4) The helmet of knights and baronets is of steel, full-faced, with the visor thrown back and without bars. (5) The helmet of esquires is always presented in profile, and is of steel with the visor closed. These distinctions are of comparatively recent date. A much greater variety of helmets is in use in Continental heraldry. A helmet is never placed over the arms of any woman except the sovereign. Sec Her.ldry. HELMET-BIRD. One of the names given to the turacos (q.v.), on account of their high, brilliantly colored crests. An Australian cocka- too, the ganga (q.v.), is called "helmet-cockatoo,' and a genus of African shrikes (Prionops) are called 'helmet-shrikes' for a similar reason. HELMET-CRAB. See King-Crab. HELMET-CREST. A South-American hum- ming-bird of the genus Oxypogon (in which there are three species) which has a tall, pointed, erectile crest suggesting a helmet-plume, and also chin-feathers resembling a long beard. See Plate of HlMMING-BlRnS. HELMET-HORNBILL. See Hoenbill. HELMET-QUAIL. A California crested quail (q.v.). HELMET-SHELL (so called from the resem- blance of the shell to a helmet). One of a genus (Cassis) of large, active gastropods, allied to the strombs, and tj'pical of the family Cassidae. Tlicy have thick, heavy shells, boldly ridged, and often handsomely colored, and some species, as the black helmet-shell of the West Indies (er-. roncously named Cassis Mada<jascareiisis) and Cassis glauta of the East Indies, are extensively used for making cameos (q.v.), because the shell is in layers of different colors. About fifty spe- cies are known, mostly Oriental ; but some in- liabit the Mediterranean and West Indian waters, and all are carnivorous, prej'ing mostly on bi- valve mollusks. HELMHOLTZ, helm'holts, Hermann von (1821-94). A German physicist, one of the most distinguished scientific men of the nineteenth century, and an authority in several departments of science. He was born at Potsdam, and studied medicine in Berlin, subsequently serving as a surgeon in the army (1843-47). He then be- came assistant in the Berlin Anatomical Museum, and a professor of physiologj', occupving chairs at Konigsberg (1849-58), at Bonn ~ ( 1855-58), and at Heidelberg (1858-71). In 1871 he became professor of physics at the University of Berlin. For several years prior to his death Helmholtz was president of the Physikalisch Technische Eeichsanstalt at Charlottenburg, and supervised the researches of this important institution. He was equally distinguished in physiology and in experimental and mathematical physics. He was selected as honorary president of the Interna- tional Congress of Electricians held at Chicago in 1893, and it was said then of him that he might have been easily first in several other departments of human knowledge quite distinct and apart from that of electricity. His physio- logical works are principally connected with the eye and the nei'vous system. As a physicist Helmholtz first attracted attention from the scientific world by a paper which he presented to the Physical Society of Berlin in 1847 on the "Conser'ation of Force." This pa])er, regarded by many as a fantastic speculation, provoked considerable discussion and established Helm- holtz's reputation. His invention of the ophthal- moscope (q.v.), an instrument which enables an observer to examine the interior of the eye, gave to the oculist a most useful device now in uni- versal use. His wonderful monograph on Seihsa- tions of Tone was first pulilished in 1863, and is the most important work on acoustics of the nineteenth century. To Helmholtz is due the generally accepted theory of color, which is that of Young (q.v.) in a modified form, wherein the perception of color depends upon the three funda- mental sensations of red. green, and violet or blue. He also developed the electro-magnetic theory of light in a manner which indicated its general possibilities. Another important research was on the theory of vortex motion, and not only did he discover the fundamental properties of such motion, but he so stated his conclusions that they have been used to explain various physical phenomena and in h"potheses concerning the con- stitution of matter and the luminifcrous ether. He was the inventor of manv ingenious methods in prneticnl phvsics. as well as new apparatus. In 1883 the German Emperor conferred upon him a title of nohilitv. He visited the United States in 1893. and died in Berlin. September 8, 1894. The first edition of Die Lchre ron den Toncmpfinflungcii, als plipsiologisclie Orundlage
