of Iridescence in Clouds. 91 



and diminish in size as the cold increases. Hence it is to be 

 presumed that the state of the atmosphere in the region of the 

 cirrus clouds will sometimes be such as to produce crystalline 

 plates of a tolerably uniform thickness. When the atmosphere 

 has been in this state at the formation of the cloud, we shall 

 accordingly have either the phenomenon of iridescence, or the 

 twin phenomenon of a vertical column of reflected light pass- 

 ing through the sun. This latter phenomenon will present itself 

 but seldom, as it requires unusual quietude in the region of 

 the cloud to allow the crystals to settle down sufficiently into 

 the horizontal position. And, accordingly, on the rare occa- 

 sions when the vertical column is seen, the cloud has been 

 observed to present a gauze-like uniformity of appearance, 

 which is an independent evidence of the calmness which is 

 essential. But it much more frequently happens that the 

 cloud is flocculent in its structure, and exposed to little breezes 

 blowing in various directions, excited by one or more of the 

 causes that have been indicated above. Whenever draughts 

 of this kind intervene, the little tabular crystals are every now 

 and then tossed about, and will then flutter ; since being flat 

 plates, subsiding through a resisting fluid, they will oscillate 

 after each such disturbance in their progress towards the 

 horizontal position. Whenever these events happen, we have 

 conditions which must result in that iridescent phenomenon 

 which is the subject of our inquiry. 



[Addition, made June 1887. — When the fluttering of the 

 lamellar crystals which form an iridescent cloud is gentle, the 

 crystals will not incline much from the horizontal position ; 

 and as this is the case which most frequently occurs, it is 

 desirable fully to consider the consequences of it. With 

 clouds of this kind the iridescent colours will be seen only 

 when the sun is low in the sky, and only in parts of the cloud 

 that are at no great distance from him. In more distant 

 clouds when the sun is low, and in all the clouds when the 

 sun is high in the sky, the flat surfaces of the crystals do not 

 become sufficiently inclined to reflect the sun's rays to the 

 spectator, and accordingly all the light which reaches the eye 

 from clouds that are so situated has been reflected by edges 

 of crystals ; or scattered in irregular ways,, and is mere white 

 light. This is the case that is oftenest seen : only a few 

 clouds near the setting or rising sun exhibit their soft iri- 

 descence. But some few times in one's life the display may 

 be seen in all quarters of the sky, and with the sun well up in 

 the heavens ; and the phenomenon is then one of the most 

 enchanting presented to us by nature. This exquisite spec- 



