524 



HALLUIN 



HALOS 



through a rare coincidence they had seen the form 

 of the dying friend ? And if this impression hap- 

 pened to be near the time when he died, is it 

 remarkable in the unscientific state of most minds 

 that they made out it was the same moment that 

 they both saw their friend's form appear and walk 

 out at the door ? When such duplicate hallucina- 

 tions are probed by hard scientific methods it is 

 always found that the hour of seeing them by the 

 two people was not quite the same, that one had 

 previously made a suggestion to the other forgotten 

 in the excitement of the moment, or that the figures 

 seen by them had on different clothing, or had 

 quite different beards. Without far more evidence 

 than has been brought forward by the pseudo- 

 scientific believers in ghosts and apparitions, an 

 age of science will never admit a hallucination to 

 be anything but a brain phenomenon, obscure per- 

 haps, but no more obscure than many other corre- 

 lated facts of brain and mind. Every advance that 

 is made in our knowledge of the brain and its work- 

 ing in relation to mind renders the rational and 

 scientific explanation of all the hallucinations of 

 the sane recorded by trustworthy, unbiassed 

 observers more easy and probable, and makes less 

 excusable the calling in to explain the facts of 

 new and unknown ' forces ' or ' influences ' in 

 nature beyond those we know and can scientifically 

 investigate. Hallucinations may be of all the 

 senses, and may be of every degree of variety and 

 complication, from flashes of light to armies of 

 men, from hummings in the ear to strains of 

 'celestial music.' But it has never been proved, 

 as ought certainly to have occurred if there was 

 any reality in those occult forces, that anything 

 has ever been seen or heard by any one which 

 the person might not possibly have seen or im- 

 agined previously, so that its image might be 

 lying registered in his brain-cells ; and no new 

 knowledge has ever come to humanity from such 

 sources. Hallucinations were much more common 

 among primitive peoples and in the early ages of 

 the world than they are now. See INSANITY. 



llallllill. a town in the French department of 

 Nord, 10 miles NNE. of Lille. Weaving of linen 

 and woollen goods, bleaching, brick-making, and 

 the manufacture of oil, chemicals, and chocolate 

 are the principal industries. Pop. 9809. 



HallHStad, a seaport of Sweden, and capital of 

 the province of Halland, on the Cattegat, 75 miles 

 SE. of Gothenburg, with trade in corn, wood, flour, 

 and coal, and salmon -fisheries. Pop. 11,825. 



Halogens, or SALT-PRODUCERS (Gr. hols, 

 ' salt ' ), are a well-characterised group of non- 

 metallic elements chlorine, bromine, iodine, and 

 fluorine which form with metals compounds 

 analogous to sea-salt. For haloid salts, see SALT. 



Halorageae, an order of thalamifioral dicoty- 

 ledons, vegetatively reduced from Onagracese 

 (q.v. ). There are about seventy known species, 

 herbaceous or half-shrubby ; universally distributed, 

 and almost all aquatic, or growing in wet places. 

 The stems and leaves often have large air-cavities. 

 The plants are insignificant in appearance, and the 

 flowers generally much reduced. None of them 

 have any important uses, except those of the genus 

 Trapa (q.v.). The only British species are the 

 Mare's Tail (Hippuris vulgar is) and the Water- 

 milfoils ( Myriophyllum ). 



Halos and Corona^. Halos are circles of 

 light surrounding the sun or moon, and are due 

 to the presence of ice-crystals in the air. The 

 commonest and usually the brightest has a radius 

 of about 22 degrees i.e. this is the angular 

 distance from the sun to its inner edge. This 

 size can be computed from the hexagonal shape 

 and known refractive power of ice-crystals. The 



calculation shows that light passing through the 

 sides of such a crystal is bent at an angle varying 

 with the direction in which it falls on the crystal, 

 but never less than 21J degrees, which is therefore 

 called the angle of minimum deviation, and in the 

 greater number of cases not greatly exceeding that 

 angle. If, therefore, the air between the observer 

 and the sun or moon be filled with such crystals 

 the light will be thrown outwards beyond the 

 angle of minimum deviation, and will mostly 

 appear at about 22 degrees distance from the 

 sun or moon, forming a circle round it. As blue 

 light is slightly more refrangible than red it is. 

 thrown farther out, and the halo appears coloured 

 red inside and blue outside. Some of the crystals 

 may, however, be lying so that the light enters 

 at a side and leaves at one end, or vice versd, 

 in whJch case the angle of minimum deviation is 

 about 46 degrees, at which distance a second 

 fainter halo is frequently seen with colours in the 

 same order as in the first. These colours are 

 generally well seen in solar halos, but not in lunar, 

 as the moon's light is too faint to give distinct 

 colour to each part. In addition to the above, a 

 third still larger halo has been seen. There are 

 only four observations of this halo on record, and 

 the radius has been estimated in the different cases 

 at from 81 degrees to 90 degrees. The cause of 

 this halo has not been ascertained. It is not 

 coloured, and may be due either to some more 

 complex form of ice-crystal or to internal reflection 

 from the hexagonal crystals. 



Another phenomenon sometimes seen with halos 

 is the Parhelic circle, which is a white circle passing 

 through the sun and parallel with the horizon. It 

 is caused by light reflected from the surfaces of ice- 

 crystals falling vertically through the air. When 

 the sun is near the horizon this circle is intensified 

 at distances of 22 degrees and 46 degrees from the 

 sun, and forms parhelia or mock-suns, and another 

 mock-sun is sometimes seen on this circle directly 

 opposite the sun. A similar circle is also formed 

 passing vertically through the sun by reflection from 

 the upper and under surfaces of the ice-crystals. 

 Halos are sometimes accompanied by contact 

 arches, which are arcs of circles touching the halos 

 of 22 degrees and 46 degrees ; they are formed by 

 long hexagonal prisms floating horizontally in the 

 air, and are curved away from the sun when it is 

 below 30 degrees altitude, but are concave towards 

 it at greater elevations. Several other more com- 

 plex forms of halo have been seen in the arctic 

 regions, but are of rare occurrence in Britain. 



Halos must not be confused with Coronce, which 

 are smaller coloured circles that appear round the 

 sun or moon when they shine through thin cloud or 

 mist. In these the red is the outermost colour, 

 and several successive sets of coloured rings are 

 usually formed. They are due to the diffraction 

 the light undergoes in passing among the drops 

 of which the cloud is composed. The radius of the 

 first red ring of a corona varies from 1 degree to 

 3 degrees, according to the size of the drops, and 

 the radii of the others are successive multiples of 

 that of the first. 



When the sun shines on a bank of fog a large 

 bow of about 40 degrees radius, resembling a rain- 

 bow, but not so brightly coloured, is seen. It is 

 often double, like the rainbow. Owing to the 

 smaller size of the water-drops in a fog than in 

 falling rain, the Fogbow is wider and fainter than 

 the rainbow. The law determining the order of. 

 the colours whether red inside or red outside 

 has not yet been thoroughly worked out. If the 

 observer is standing on an elevated point so that 

 his shadow falls on the fog, coloured rings called 

 Glories or Anthelia are often seen. Five or six 

 sets of colours have been observed, the outermost 



