HELICON. 734 HELIOPHYLLXJM. helicon is built in the form of a circle and carried around the body. I'or this reason it is generally used in military bands in preference to the tuba, Ijecause it can be carried with less effort when inarching. HELICONID^. A family of butterflies noted in the study of mimicry. See ^Iimicby. HE'LIOCEN'TRIC (from Gk. ijios, hclios, sun + nivrpov, kcniron, centre). A term in as- tronomy, signifying that the sun is taken as the centre of reference or view. It is opposed to geocentric (q.v. ), which indicates that the earth is taken fur centre. HEL'IODO'BUS (Lat., from Gk. 'HXi65upos). A Greek romance-writer, born at Emesa, in Syria. He flourished, probably, at the end of the third century A.U., and is not to be con- fused with Heliodorus, the Bishop of Trikka in Thessaly, who, according to the Church his- torian Socrates, wrote a romance in his youth. His work, in ten l)Ooks, entitled .Hthwpiru { XldmTTLKa] ■, narrates the loves of Theagenes anol Cliaricleia. At times the work shows almost epic beauty and simplicity. The descriptions are excellent, and the interest of the reader is in general well maintained; but in the presentation of the emotions the work is far less successful. The language betrays the Semitic origin of the author, and the style shows his rhetorical train- ing and adherence to the Neo-Pythagorean School. Heliodorus was strongly influenced by Homer and Euripides, and in his turn has been a favorite model of many French dramatists — for example, of Kacine (q.v.). There are editions by Bekker (Leipzig, 1855), and Hirsehig (Paris, 1856) ; English translations by Rowland Smith (London, 1855) and Underdowne (1857; last printed London, 1895). Consult Rhode, Der griechische Roman (Leipzig. 1900). HE'LIODO'RXJS, treasurer of Seleucus IV. of Syria (B.C. 187-175). Seleucus sent him to Jerusalem to rob the temple, but he was refused admittance by Onias. the high priest, on the first day. and on the next was driven away, accord- ing to the story ( II. Mace. iii. ) , by a terrible angel. He murdered his master and usurped the thrcne. from which he was driven by Attains and Eumenes, princes of Pergamus, after a few months. HE'LIOGAB'ALTJS. See El^vgabalus. HELIOGRAPH (from Gk. ■fjXios, /leiios, sun + ypa<t>la, graphia, writing, from ■ypa.<f>eii>, gra- phcin, to write). An instrument used for com- munication between distant stations by reflec- tions of the sun from a mirror or system of mir- rors. The great advantage of this method over the ordinary signal system is that the appara- tus is more portable and can be used over greater distances, but only with success in regions w-here the atmosphere is clear of clouds for considerable periods of time. There are two methods of using the heliograph, which are based in the main on the dot and dash : the reflection may be obscured except when the screen is temporarily removed to produce a flash or letter ; or the reflection may be kept exposed except when it is obscured to produce a letter. The first method is said to be the easier for the beginner, but the second less fatiguing to the eye. The distance through which this mode of communication may be car- ried on varies with the size of the mirrors and the clearness of the atmosphere. In 1890 mes- sages were signaled along the Arizona mountains fur 215 miles. When the signaling station forms an angle greater than a right angle between the sun and the receiving station, two mirrors are used to prevent too great a loss of rays by oblique reflection. The mirrors are mounted on tripods, and are held by a socket, or a universal joint. Besides its use as a signaling instrument, the heliograph has served to define distant points in a geodetic survey, and for this purpose has been employed in trianguhitiun. Perliaps the chief use of the heliograph is in military operations in the field. It formed the sole means of communication between the besieged British garrisons and the relief columns during the Boer- British War. HELIOGRAPHY. See PnOTOORAPHT. HELIOLITES, he'li-6-li'tez (Neo-Lat., from Gk. ijXios, hClios, sun + l6oi, lithos, stone). A genus of fossil alcyonarian corals found in Orduvician and Silurian rocks and less abun- dantly in the Devonian formations. Heliolites is the type of an important family, Heliolitidir, comprising about 10 genera and 45 species. All the members of the family have rounded coral masses that consist of larger tubes surrounded by smaller tubes. The genera and species are based upon modifications of the horizontal and vertical walls that traverse the interior of the mass. They have been found in all countries where Silurian and Lower Devonian formations exist and they often form fossil coral reefs of consid- erable extent. Heliolites intcrstinctus. the best- known species, is of world-wide distribution. Consult Lindstrum, "Remarks on the Helioli- tid;p," in Kongllga Srenska Vetenslcaps-Akade- miens Flandliiifldr, vol. xxxii. (Stockholm, 1899). See Coral; Octocoealla. HEXIOM'ETER (from Gk. TiXios, hclios, sun + iiiTpov, tnrtrun, measure). An instrument originally intended for measuring the angular diameter of the sun. It is a telescope in which the object-lens is in two halves, each of which will form a perfect image in the focus of the eye- piece; and the images may be made to diverge, coincide, or overlap each other by varying the distance between the half-lenses. If the diameter of the sun is to be measured, the two lenses are adjusted so that the images will touch each other, then the distance between the centres of the two object-glasses measured in seconds gives the diameter of the sun. Fraunhofer made many remarkalde improvements in the heliometer, but the perfected type of instrument as used to-day is the work of" A. Repsold & Sons of Hamburg. The original iise of the heliometer has been greatly extended, especially in the hands of Gill at the Cape of Good Hope Observatory. As now made, angular distances above two degrees can be meas- ured on the sky with extreme precision. On the whole, the modern heliometer is considered the most precise measuring instrument known to astronomy. HE'LIOPHYL'LUM (Xeo-Lat., from Gk. ■ijios, hclios. sun + tjivWov, phj/Uon. leaf). A fossil coral very common in the Hamilton shales of central and western New York and other lo- calities in America. This coral is usually simple, of conical form, with a large shallow cup in which a great number of slender septa or w'alls radiate from the centre toward the elevated edge.
