OPTICS. 



53 



rally accompanied with parhelia,* or 

 mock suns, which appear at the places 

 where two halos, or arches of luminous 

 circles, intersect each other. 



The large white halo, called in Scot- 

 land a brough, generally appears round 

 the moon in cold \veaiher, when the sky 

 is of an uniform misty tint ; and the 

 prismatic halos, generally called coronce, 

 which are seen round the sun and 

 moon, are commonly seen in fine wea- 

 ther, when white, thin, fleecy clouds 



Fig. 51. 



float in the atmosphere. Owing to the 

 dazzling effect of the sun's rays, the 

 halos which surround his disk may be 

 seen to most advantage when he is 

 seen by reflexion in a pool of water. 



One of the most curious and best 

 described combinations of halos and 

 parhelia was observed by Hevelius, at 

 Dantzic, on Sunday, the 20th February, 

 1661, New Stile. It is represented in 

 fig. 5 1 , and has been thus described by 

 Hevelius.* 



" A little before eleven o clock, the sun 

 being towards the south, and the sky 

 very clear, there appeared seven suns 

 together, in several circles, some white 

 and some coloured ; and these with very 

 long tails, waving and pointing from the 

 true sun, together with certain white 

 arches crossing one another. 1st. The 

 true sun at A, being about 25 high, 

 was surrounded almost entirely by a 

 circle whose diameter was 45, and 

 which was coloured like the rainbow, 

 with purple, red, and yellow, its under 

 limb being scarcely 2i' above the hori- 

 zon. 2d. On each side of the sun, at B 

 and C, towards the west and east, there 

 appeared two mock suns coloured, 

 especially towards the sun, with very 

 long and splendid tails, of a whitish co- 

 lour, terminating in a point. 3d. A far 

 greater circle, Y X H V Z, almost 9C 

 in diameter, encompassed the sun, and 



the former lesser circle, G B I C, and 

 extended itself down to the horizon. 

 It was very strongly coloured in its 

 upper part, but was somewhat duller and 

 fainter on each side. 4th. At the tops 

 of these two circles, at G and H, were 

 two inverted arches, whose common cen- 

 tre lay in the zenith, and these were 

 very bright and beautifully coloured. 

 The diameter of the lower arch, Q G R, 

 was 9G, and that of the upper one, 

 T H S, was 45. In the middle of the 

 lower arch at G, where it coincided with 

 the circle B G C, there appeared another 

 mock sun ; but its light and colours 

 were dull and faintish. 5th. There ap- 

 peared a circle, B C E F D, much bigger 

 than the former, of an uniform whitish 

 colour, parallel to the horizon, at the 

 distance of 25, and 130 in diameter, 

 which arose, as it were, from the colla- 

 teral mock suns B and C, and passed 



9 From two Greek words bonifying near the Sun. * Appendix to bis Mcrcvrius in Hole Fisus, p. 



