OPTICS. 



55 



with a sort of continued cloud, but so 

 thin that the colour of the blue sky 

 appeared through it. The wind blew 

 very gently from the north."* 



Our limits will not permit us to give 

 any farther description of individual 

 halos ; but the inquiring reader will 

 have his curiosity amply gratified by 

 consulting the article Halo, in the 

 Edinburgh Encyclopedia, vol. x. p. 

 6 1 '1 ; and he will find in the accounts 

 of the recent voyages of Captains Ross, 

 Parry, and Scoresby, descriptions of 

 several which have been observed since 

 the publication of the above article. 



The explanations which have hitherto 

 been given of this class of phenomena, 

 are by no means so satisfactory as 

 miirht have been expected in the pre- 

 sent improved state of optical science. 

 It seems, however, to be satisfactorily 

 proved, that they owe their origin to 

 the crystals of ice and snow floating in 

 the atmosphere, and in some cases to the 

 action of drops of rain of different sizes. 



That crystals of ice do float in the 

 atmosphere is well established. Sir 

 Charles Giesecke, who lived seven years 

 in Greenland, describes this phenomenon 

 in the following words : " Previous to 

 that operation of nature, (viz. the freez- 

 ing of the sea,) the sea smokes like 

 burning turf-land, and a fog or mist 

 arises, called frost smoke. This cutting 

 mist frequently raises blisters on the 

 face and hands, and is very pernicious 

 to the health. It appears to consist of 

 small particles of ice, and produces the 

 sensation of needles pricking the skin."t 



The existence of such crystals in the 

 arctic regions being thus proved, there 

 can be little doubt that they occur in 

 the upper part of our own atmosphere, 

 where the cold is sufficient to freeze the 

 watery particles of which the clouds and 

 vapours are composed. That a number 

 of transparent crystals placed between 

 the eye and a luminous body, will pro- 

 duce halos round that body, whose 

 diameters will depend on the refractive 

 power, and the refracting of the crystals, 

 may be proved by the following ex- 

 periment, described by Dr. Erewster in 

 the article Curiosities of Science, in 

 the Edinburgh Encyclopaedia, vol. xvii. 

 p. *590. If we spread a few drops of a 

 saturated solution of alum over a plate 

 of glass, it will quickly crystallize, co- 

 ig the glass with an imperfect crust, 



* Hr.genii Opera Posthuma, p. 366. 

 t Arucle Greenland, in the Edinburgh Encyclo* 

 pcedia, vol. x. p. 489, col. 2. 



which consists of flat, octohedral cry- 

 stals scarcely visible to the eye. When 

 this plate is held between the sun or any 

 other luminous body and the observer, 

 whose eye must be kept close behind the 

 smooth side of the glass plate, he will 

 see three fine halos encircling the lu- 

 minous body at different distances. The 

 innermost halo, which is the whitest, is 

 formed by the refraction of the rays of 

 the sun through the pair of faces of the 

 octohedral crystals which are least 

 inclined to each other. The second 

 halo, which is more coloured, having 

 the blue rings outwards, is formed by 

 refraction through a pair of faces more 

 inclined to each other ; and the third 

 halo, which is very large and highly 

 coloured, is formed by a pair of faces 

 constituting a prism, with a still greater 

 refracting angle. Now each individual 

 crystal of the alum forms, by means of 

 three of the similar prisms which it 

 includes, three images of the sun, placed 

 at points 120 distant from one ano- 

 ther, and in the circumference of a 

 circle of which the sun is the centre ; 

 and as the numerous minute crystals 

 with which the plate of glass is covered, 

 have their refracting faces turned in 

 every possible direction, the whole cir- 

 cumference of each halo will be filled 

 up as it were with images. Similar 

 effects may be obtained with other 

 crystals ; and when they have the pro- 

 perty of double refraction, (which alum 

 has not,) each halo will be either 

 doubled when the double refraction is 

 considerable, or rendered broader when 

 the double refraction is small. 



Having thus shown how circles of 

 light may be formed by viewing a lu- 

 minous body through a number of 

 minute crystals, we shall proceed to 

 give a brief sketch of the leading 

 opinions which have been entertained 

 respecting the cause of halos. 



Although Descartes had stated that 

 halos were produced by crystals of ice, 

 yet it was Huygens who first investigated 

 the form of the crystals, or rather 

 masses of hail, which was necessary to 

 produce the observed phenomena. He 

 supposes that there are globular par- 

 ticles of hail not larger than turnip 

 seed, the outer portion of which is 

 melted and in the state of water, while 

 the inner part or kernel is opaque like 

 snow. These globules, he thinks, were 

 first globules of soft snow, which are 

 rounded by a continual agitation in the 

 air, and thawed on the outside by the 



