640 PHILOSOPHICAL TRANSACTIONS. [aNNO 1780. 



To the above description of the image it may be necessary to add the following 

 particulars which attended it. The mist was very thick, near the surface of the 

 meadows, though rarer ujiwards, and chiefly, if not solely, on the side of the 

 hill opposite to the sun. The place where Mr. C. stood was just on its confines; 

 and as he advanced into it, the object became gradually fainter and fainter. As 

 the sun dispersed the vapour, the appearance faded proportionably; and about 

 half an hour after he first saw it, it was scarcely visible. The evening before was 

 wet ; but the drops on the hedges were congealed by frost. Where the sun 

 shone the bushes were each invested with a mist, as if owing to the vapours ex- 

 haled from them by the sun's warmth ; and, on a nearer inspection, he could 

 clearly discern the little humid ' particles which occasioned it, and v/hich were 

 floating around the bushes at about half an inch distance from each other. 



Such were the most material circumstances of this beautiful and remarkable 

 appearance. We have only 2 instances of a like kind mentioned in Dr. Priestley's 

 History of Light and Colours. The first is given by M. Bouguer as seen on 

 the Andes; and the other by Dr. Macfait as seen in Scotland. A 3d however 

 may be met with as observed at Pambamarca, in Ulloa's Voyage to South 

 America. 



With regard to the elliptical form of the curve spaces, as it cannot be ac- 

 counted for from refraction, it is probably owing to the oblong figure of the ob- 

 server's shadow, which is very evidently the dark part in the middle, and to which 

 the coloured marginal rings are in some sort obliged to conform. The bright 

 places AA correspond to an appearance once observed by Dr. Smith, and which 

 he very plausibly attributes to a confused mixture of the principal reflected beams 

 that exhibit the ordinary bow. In his Treatise of Optics there is an account o! 

 Dr. Pemberton's theory for the slender rings of colours, which are sometimes 

 seen within the rainbow, which Dr. Langwith first described in the Philos. 

 Trans., N° 375, (Abrid. vol. 6, p. 623) and from which some idea may be 

 formed of the cause of the coloured part of the image. It is in substance this: 

 if the drops of rain, &c. which the sun shines on be exceedingly small, from 

 the irregular reflection of all surfaces, and the fits of easy transmission, which 

 the dissipated rays may undergo in their passage through those little globules, 

 there may naturally be formed other coloured arches within the common bow for 

 a number of successions. Hence, with regard to the instance in question, since 

 its rings were so very small in diameter, it appears, that on some account or 

 other the refracted, coloured, and dissipated rays alluded to, have in their return 

 to the eye nearly made the smallest angles possible with the lines ot in- 

 cidence. 



