CL 63%) 
XIII.—ON THE CAUSE OF IRIDESCENCE IN CLOUDS. By G. JOHNSTONE 
STONEY, M.A., D.Sc., F.R.S.; a Vice-Presipent, R.D.S. 
[Read, February 16, 1887, except the last four paragraphs, which were read March 23, 1887. ] 
Wuen the sky is occupied by light cirro-cumulus cloud, an optical phenomenon of 
the most delicate beauty sometimes presents itself, in which the borders of the 
clouds and their lighter portions are suffused with soft shades of colour lke those 
of mother-of-pearl, among which lovely pinks and greens are the most conspicuous. 
Usually these colours are distributed in irregular patches, just as in mother-of- 
pearl ; but occasionally they are seen to form round the denser patches of cloud a 
regular coloured fringe, in which the several tints are arranged in stripes following 
the sinuosities of the outline of the cloud. 
I cannot find in any of the books an explanation of this beautiful spectacle, 
all the more pleasing because it generally presents itself in delightful summer 
weather. It is not mentioned in the part of Moigno’s great ‘ Répértoire 
d'Optique,” which treats of meteorological optics, nor in any other work which I 
have consulted. It seemed desirable, therefore, to make an attempt to search 
out what appears to be its explanation. 
At the elevation in our atmosphere at which these delicate clouds are formed, 
the temperature is too low, even in midsummer, for water to exist in the liquid 
state; and, accordingly, the attenuated vapour from which they were condensed 
passed at once into a solid form. They consist, in fact, of tiny crystals of ice, not 
of little drops of water. If the precipitation has been hasty, the crystals will, 
though all small, be of many sizes jumbled together, and in that case the beautiful 
optical phenomenon with which we are now dealing will not occur. But if the 
opposite conditions prevail—which they do on rare occasions—if the vapour had 
been evenly distributed, and if the precipitation took place slowly, then will the 
crystals in any one neighbourhood be little ice crystals of nearly the same form 
and size, and from one neighbourhood to another they will differ chiefly in number 
and size, owing to the process having gone on longer or taken place somewhat 
faster, or through a greater depth, in some neighbourhoods than others. This will 
give rise to the patched appearance of the clouds which prevails when this pheno- 
menon presents itself. It also causes the tiny crystals, of which the cloud consists, 
to grow larger in some places than others. 
Captain Scoresby in his Account of the Arctie Regions gives the best description 
of snow crystals formed at low temperatures with which I am acquainted. From 
TRANS. ROY. DUB. SOC., N.S. VOL. II. AR 
