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Notes on Iridescent Clouds. 



By W. A. JoxEs. 



[Read May 5, 1885.J 



In 8ome of the scientific journals attention has been lately 

 drawn to the phenomenon of iridescence in clouds in daytime. 

 The mass of correspondence on the subject may be assumed to 

 have been started by the great amount of interest lately 

 excited all over the world by the beautiful sunrises and sun- 

 sets, with the accompanying theories which have been pro- 

 pounded to account for them. 



There has thus been a large amount of contemporaneous 

 observation lately directed towards all meteorological pecu- 

 liarities. Many of the observers have seen things new to 

 -them, and have recorded their observations as original, whilst 

 in some cases the appearances observed have been misnamed, 

 and the term "iridescent clouds" has been frequently applied 

 to tints belonging to an entirely different class of sky colours. 



I shall endeavour to show by evidence that the appearances 

 which some observers have referred to unusual conditions are 

 not new or unusual, but that they have been observed a long 

 time past, and are of frequent occurrence, though they often 

 escape detection. 



It will be seen from the descriptions by others and myself 

 that the clouds under notice are similar to those observed by 

 Herschel in 1841, and described in his meteorology, I shall 

 quote Herschel first. He says : — •" The clouds were tinged 

 with bands of colour, not in circular form around the sun, but 

 following the sinuosities of the clouds. The colours com- 

 menced from the white area forming the interior, and pro- 

 ceeding outwards to the edge, were — 1st, white ; 2nd, very 

 pale pink ; 3rd, blue green ; 4th, at the edge, purplish pink. 

 These colours obviously had no reference to the greater or 

 less proximity of the clouds to the sun. but depended upon 

 the thickness of the cloud or the length of the path within it 

 traversed by the visible rays. After watching for some time 

 the bands grew broader and the tints stronger, with a ten- 

 dency to form a corona. It seems impossible to regard these 

 colours otherwise than as the resultants of the superposition 

 of a series of interference fringes following a regular pro- 

 gression of breadth (due to a progressively increasing size of 

 the drops) from the exterior to the interior of the clouds." 



Similar clouds to these were observed and recorded at the 



