1852.] 



tlic colours were most distinct, being in those parts towards tlio 

 Sun of a deep orange, but on the inner boundary of the two 

 latter arcs of an indigo blue, the intervening space of a hght 

 green." 



The radius of the inner ring, as measured by Captain Lefroy, 

 at Toronto, was 22 ° 53'. 



In another lialo, observed February 9, 1851, by Mr. James S. 

 Clouston, C. T., at Moose Factory, we find " a horizontal circle 

 of a whitish colour passing through the sun ; a halo round the 

 sun of about 22 ° radius, at the point of intersection of which 

 with the former were two very bright mock-suns ; a second halo 

 of about 45 '-' radius, family tinged with the prismatic colours, 

 (and as in the previous one) two arcs touching them at their 

 hifdiest pohits, convex to the sun, both coloured, but the colours 

 of the second being very vivid Hke a rainbow; on the horizontal 

 ch-cle, two faint elongated mock-suns, each about 120 ° (90 ° ?) 

 on either side of the sun, and a third dircctlj' opposite to the sun, 

 and very much elongated." Captain Back describes a lunar halo 

 in which a white cross passed through the moon bounded by a 

 halo of 22 ° , and ha\iug a mock moon in the end of each 

 branch of the cross. In a halo of the sun, observed by Hevelius 

 in 1661, A. D., no less than six mock-suns were seen, as also in 

 another observed by M. Lambert, in June, 1838. At Kiaihta, 

 in Siberia, on February 4, 1829, at sunrise, were seen luminous 

 rays issuing on both sides of the sun, (known in that country 

 by the name of the sun's ears,) \\hich extended gradually till 

 they made the complete tour of the horizon, forming a circle in 

 the circumference of which were situated seven mock-suns. But 

 of all recorded halos, the most complex and gorgeous is the one 

 seen at Gotha, on May 12, 1824, and of which a full account 

 may be found in the Ency. Met. Art. Meteorology. 



It will be now seen that the term halo indicates a phenomenon 

 so complicated as to render its description difficult, no observer 

 having ever yet seen it complete, and the appearances often 

 changing during the time of observation ; however, the principal 

 paiis of it may be thus defined : — 



1. A horizontal white chcle passing through the sun and 

 making the complete cu'cuit of the heavens. 



2. A vertical white cu-cle, also passing through the sun and 

 terminated by the horizon. 



These are produced by reflection of the sun's ra3's, the former 

 at the faces of the ice-crystals, whose axes are veilical and at the 

 bases of those with axes horizontal ; the latter at the faces of the 

 prisms, whose axes are horizontal and perpendicular to the verti- 

 cal plane through the sun and spectator. The crystals tending 

 to arrange themseb'es according to the law of least resistance, 

 will naturally be found in greater abundance with their arcs 

 horizontal or vertical, than in any other position. 



3. These two circles form the white cross in Captain Back's 

 lialo, and at their other point of intersection give rise to a pale 

 and vivid mock-sun, which is consequently just in the opposite 

 point of the heavens to the real sun and at the same height abo-\"G 

 the horizon. 



4. Two other wliite circles of verj- I'are occurrence passing 

 also through the sun and the above-mentioned image, and equally 

 inclined, though at a variab'e angle, to the vertical circle (2.) 

 These arise from reflection at the faces of prisms when a 

 considerable number of them happen to have their axes inclined 

 at the same angle to the vertical. 



5. Three circular coloured ring's, or kcdos i^i'oper, surrounding 

 the sun and having radii about 22, 46, and 90 degrees respectively. 

 They all display the prismatic colors, the two interior having red 

 on the inside and violet without, (these being distinguished at a 

 glance from coronce or where the order of colour is the reverse), 



ON THE ATMOSPHERIC PHENOMENA OF LIGHT. 



27 



the third or outermost on the contrary has violet within and red 

 without. Inside the first halo is comparative darkness, but on 

 the outside its \ioIet fades away into the azure of the sky, succeeded 

 again by a darker space just within the second halo, between which 

 and the third is considerable illumination followed by darkness 

 outside the third. Of these the first is produced by rays refracted 

 through two adjacent faces of the pilsras in such a position that 

 the refracted ray undei'goes the le;ist deviation ; the second by 

 those refracted in like manner through a face, and issuing through 

 the base, and the third by rays issuing after one internal reflection 

 exactly as in the primary rain-bow. The residts of theory agree in 

 all particulars with regard to these, with the observed tacts, but 

 there is mention made in one of Captain Pan-y's halos of a 

 prismatic circle of 38° radius, and in a halo seen at the Observatory 

 in Toronto, on March 9, 1841, of one of 30° radius ; if these 

 were not sinqsle coronce (the order of colours is imfortunately 

 not mentioned in either case), and the measurements be accurate, 

 they must be regarded as facts yet unexplained, and it appears 

 very difficult to frame any hypothesis for them. 



6. Two circular coloured axes, generally of great brilliancy, 

 touching the first halo at its highest and lovv'est points, and 

 turning their convexity towards the sun, being red outside and 

 violet within. These are formed in the same manner as the first 

 halo, by prisms with axes horizontal, and occurring in great 

 numbera. The circles will have the zenith for their centre, their 

 apparent diametera varying with the sun. 



7. Two precisely similar rings, touching the second halo in its 

 highest and lowest points, produced, according to M. Galle, in 

 the same manner as tlie second halo, by piisms whose axes are 

 vertical ; but this explanation does not seem satisfactory. 



8. Two other circles — very rare — touching the second halo at 

 points distant 60° from its lowest point, of which no account has 

 yet been given by theorj". 



9. Lastlj', at, or near, all the intersections of the halos proper 

 with the vertical and horizontal circles (1 and 2), have been 

 observed, at one time or other, images of the sun, mock-sun, or 

 parhelia as they are called, in number eleven. Those of the first 

 and second halo generall}' appear coloured like the halos 

 themselves, and spread out tails tending away from the sun 

 along the white circles ; but in those of the third halo, which 

 are extremely rare, the coloui-s have never been seen, owing to 

 the faintness of this halo. They occur generally a httle outside 

 of the exact intersection of the circles, which is successfully 

 explained by Venturi, from the fact of the refraction not taking 

 place exactly in a plane perpendicular to the edge of the prisms. 



There may be other varieties of the halo which ar-e not included 

 in the above enumeration; and, indeed, the vast diversity of 

 forms which the ice-crystals may take, and the great number of 

 ways in which they may present themselves to the sun, evidently 

 oft'er a complication of circumstance that baflles analysis, or 

 description. There is still a class of phenomena depending on 

 the atmospheric polarization of light whose examination must be 

 reserved for a future occasion, and also a few of which it may be 

 doubted whether they are due to atmosjiheric action ; thus, not 

 to mention the auroral arch and zodiacal light which some liavo 

 attempted to resolve into atmospheric phenomena, the dark lines 

 of the solar spectrum and the twinkling of the stars ma}', with 

 much probability, be thus referi'ed. Of the former of these two, 

 no attempt even at explanation has yet been made, and the latter 

 has been the crux of optical science for ages. It consists, as may 

 be seen any clear evening, in the star undergoing rapid changes 

 of intensity and coloiu' — ■ 



"The fiery Siriiis alters here, 



And bickers into red and emerald ." 



Philosophers, from Aristotle down to Newton, have tried tlieir 

 hauls in framing hypotheses, more or less ingenious, to account 



