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OPTICS. 



through three other parhelia, of an uni- 

 form whitish colour, like silver : one at 

 D, almost 90 from the true sun, towards 

 the east; another at E, towards the 

 west ; and a third at F, in the north, 

 diametrically opposite to the true sun, 

 all of the same colour and brightness. 

 There passed also two other white arches, 

 E N, D P, of the greatest circle of the 

 sphere, through the eastern and western 

 mock suns E, D, and also through K, 

 the pole of the ecliptic. They went 

 down to the horizon at N and P, cross- 

 ing the great white circle obliquely, so 

 as to make a white cross at each par- 

 helion ; so that seven suns appeared very 

 plain at the same time ; and if I could 

 have seen the phenomenon sooner from 

 an eminence, I do not question but I 

 should have found two more at H and I, 

 which would have made nine in all ; for 

 there remained in those places such marks 

 as made this suspicion not improbable. 



" This most delightful and extraordi- 

 nary sight lasted from 30 minutes past 1 

 to 5 1 minutes past 1 1 ; though it had not 

 the same appearance all that while, but 

 sometimes one and sometimes another. 

 It appeared in the perfection of this de- 

 scription at about 11 o'clock, and then 

 degenerated by degrees. The northern 

 mock sun at F vanished first of all, 

 together with a part of its circle ; the 

 other parhelia, with their arches, lasted 

 till 10 minutes past 1 1 ; then the eastern 

 mock sun, and after that the western, 

 vanished, with both the crosses. Soon 

 after this the collateral parhelia C, D 

 suffered several changes ; sometimes one 

 was brighter than the other, in light 

 and colours, and sometimes fainter and 

 darker. For at 18 minutes past 10 the 

 eastern parhelion at C vanished, while 

 the western parhelion at B remained 

 very conspicuous ; and at 24 minutes 

 past 1 1 the eastern one was very bright 

 again, and remained so, while the west- 

 ern one disappeared at 40 minutes past 

 1 1 ; although this western one had al- 

 most always the longer tail. For the 

 tip of it was frequently extended for 30 

 degrees, and sometimes 90, as far as the 

 parhelion E ; but the tail of the eastern 

 one C was scarcely above 20 degrees. 

 At 30 minutes past 11 the great ver- 

 tical circle, Y X H V Z, was destroyed ; 

 but the inverted arches H and G, to- 

 gether with the collateral parhelia B 

 and C, continued to the last. 



" The figure of this phenomenon is 

 drawn in the same manner as the con- 

 stellations are drawn upon an artificial 



globe, to be viewed by the eye on the 

 outside of it. Vor by this means every 

 thing is represented much clearer and 

 clistincter. Nevertheless, the place of 

 the observer was nearly under the ze- 

 nith within the circle, parallel to the 

 horizon ; so that the true sun appeared 

 to him in the meridian, the mock sun F 

 in the north, and the other two at D 

 and E on each hand. But if you desire 

 to have this extraordinary phenomenon 

 represented a little plainer ; upon an ar- 

 tificial globe, whose pole is elevated to 

 our latitude at Dantzic, with the centre 

 A in the 2d degree of Pisces, where the 

 sun then was, and with a semidiameter 

 of 22 degrees, describe the circle 

 G B I C ; 2, and then the circle Y X H 

 Y Z, with a radius of 45 degrees ; 3. 

 and with the same centre and semi- 

 diameter of 90 degrees, draw the circle 

 N E K D P through the two white mock 

 suns E, D ; 4. and with a semidia- 

 meter of 22 , the zenith being the centre, 

 draw the arch I H S ; 5. and also the 

 arch Q G II, with a radius of 90 degrees, 

 upon the same centre; G. and, lastly, 

 the circle BEFDC parallel to the 

 horizon, with a radius of 90 degrees. 

 And the draught being finished in this 

 manner, will appear very beautiful and 

 harmonious." 



In the drawing of this phenome- 

 non, the halos are represented as cir- 

 cles, with the sun in their centre ; but 

 they are in : general of an oval form, 

 wider below man above, and having the 

 sun nearer their upper than their lower 

 extremity. This is an optical illusion, 

 depending on the apparent figure of the 

 sky. When the halo touches the hori- 

 zon, its apparent vertical diameter has 

 been estimated by Dr. Smith as divided 

 by the moon in the proportion of about 

 two to three, or four, and is to the hori- 

 zontal diameter drawn through the moon 

 as four to three nearly. 



A halo of a different kind, and exhi- 

 biting all the prismatic colours, was 

 observed by Mr. Huygens on the 1 3th 

 of May, 1652. " I observed," says 

 he, " a circle about the sun as its 

 centre : its diameter was about 46, arid 

 its breadth the same as that of a com- 

 mon rainbow. It had also the same 

 colours, though very weak, and scarcely 

 discernible, but in a contrary order ; the 

 red being next the sun, and the blue 

 being very dilute and whitish. All the 

 space within the circle was possessed by 

 a vapour duller than the rest of the air ; 

 of such a texture as to obscure the sky 



